At issue in this case is whether the voters of Haverhill validly rescinded an amendment to that city’s zoning *726ordinance by a referendum. The city council of Haverhill (council) voted in August and September, 1986, to amend c. 255, § 87, of the Haverhill zoning code, and to grant a special permit to A.J. Lane & Company, Inc. (Lane), to build a proposed planned unit development (PUD).3 The PUD, to be known as Haverhill Green, was to include a hotel, motel, and conference center. The special permit was expressly conditioned on the zoning amendment, without which the PUD would not conform to zoning ordinance in effect. The plaintiffs, a group of landowners whose properties abut the site of the proposed PUD, filed a complaint in Superior Court under G. L. c. 40A, § 17, G. L. c. 214, § 1, and G. L. c. 231A, § 1, to appeal from the council’s decision. They alleged that the proposed PUD would “not only ‘impair the integrity’ of the zoning district” but would “transform it entirely.” At the same time, they initiated a referendum question pursuant to G. L. c. 43, § 42. As the council did not rescind the amendment, see § 42, it was referred to the voters for a special city election. The voters of Haverhill rejected the zoning amendment in November, 1986, by a two-to-one margin. The council then voted to invalidate the special permit. The plaintiffs moved for summary judgment, and Lane moved for partial summary judgment. A Superior Court judge allowed the plaintiffs’ motion and denied Lane’s motion, concluding in a comprehensive memorandum of decision that a referendum could rescind an amendment to a *727zoning ordinance.4 Pursuant to G. L. c. 231 A, he declared that the amendment to the zoning ordinance was rescinded and that Lane’s special permit was null and void as a result of the referendum. Lane appeals. We transferred the case to this court on our own motion. We affirm.
Lane argues that amendments to zoning ordinances are not subject to repeal by referendum. Alternatively, Lane asserts that the referendum cannot retroactively affect the validity of a special permit issued prior to the referendum, while the enabling amendment was in effect. Lane therefore asks that we reverse the judgment of the Superior Court and order partial summary judgment in its favor, with a declaration either that G. L. c. 43, § 42, does not apply to repeal of an amendment to a zoning ordinance, or, in the alternative, that the validity of the special permit was unaffected by the referendum in the circumstances of this case.
1. The application of the referendum to a zoning ordinance. Lane contends that G. L. c. 40A, § 5, as amended through St. 1984, c. 189, § 47,5 provides the only means by which a zoning ordinance may be changed or repealed. Because G. L. c. 40A, § 5, applies specifically to zoning ordinances, Lane argues that it takes precedence over the general provision for repeal by referendum, contained in G. L. c. 43, § 42. As the procedures set forth in c. 40A, § 5, do not include referenda, Lane concludes that the referendum was ineffective to repeal the amendment to the zoning ordinance.
By its terms, G. L. c. 43, § 42, applies to “any measure, except a revenue loan order, [passed] by the city council.” The *728statute provides that, if twelve per cent or more of the registered voters in a city file a protest against any such a measure, the measure “thereupon and thereby is suspended from taking effect.” The city council must then “immediately reconsider” the measure. If the city council does not vote to rescind the measure, it must be submitted to the voters, “and such measure . . . shall forthwith become null and void unless a majority of the registered voters voting on the same . . . vote in favor thereof.” Id.
General Laws c. 40A, § 5, reads in pertinent part: “Zoning ordinances . . . may be adopted and from time to time changed by amendment, addition or repeal, but only in the manner hereinafter provided.” The remainder of the section sets forth the means of adoption or change of zoning ordinances, with specific provisions concerning notice, hearing, and two-thirds or three-fourths votes. It provides further that a zoning ordinance is effective from the date of adoption or amendment. Because the section makes no mention of the use of a referendum to repeal a zoning amendment, Lane concludes that the zoning statute occupies the entire field in amending or repealing zoning ordinances and does not permit a referendum.
“[W]here two or more statutes relate to the same subject matter, they should be construed together so as to constitute an harmonious whole consistent with the legislative purpose.” Registrar of Motor Vehicles v. Board of Appeal on Motor Vehicle Liab. Policies & Bonds, 382 Mass. 580, 585 (1981). “[W]e assume, as we must, that the Legislature was aware of the existing statutes” when it enacted the subsequent statute. Hadley v. Amherst, 372 Mass. 46, 51 (1977). “We will find an implied repeal of one statute by another only when ‘the prior statute is so repugnant to, and inconsistent with, the later enactment that both cannot stand. ’ ” Boston v. Board of Educ., 392 Mass. 788, 792 (1984), quoting Commonwealth v. Graham, 388 Mass. 115, 125 (1983).6 Implied repeal of a *729statute is not favored. See Dedham Water Co. v. Dedham, 395 Mass. 510, 518 (1985); Cohen v. Price, 273 Mass. 303, 308 (1930).
The referendum statute, by its terms, applies to “the final passage of any measure, except a revenue loan order, by the city council.” G. L. c. 43, § 42. The word “measure” is defined in G. L. c. 43, § 37, “[to] mean an ordinance, resolution, order or vote passed by a city council.” Thus, on its face, the referendum statute encompasses zoning ordinances, and Lane has not argued to the contrary. See Gorman v. Peabody, 312 Mass. 560, 562-565 (1942). Cf. Fantini v. School Comm. of Cambridge, 362 Mass. 320 (1972) (vote of school committee not to reappoint superintendent of schools not a “measure” subject to referendum); Dooling v. City Council of Fitchburg, 242 Mass. 599, 602 (1922) (city council orders directing mayor to execute construction contracts not “ ‘measures’ within the jurisdiction of the city council,” hence, not subject to referendum). The fact that the Legislature specified one exception (for revenue loan orders) strengthens the inference that no other exception was intended, Iannelle v. Fire Comm’r of Boston, 331 Mass. 250, 252 (1954). General Laws c. 43, § 42, embodies an important democratic value and “should be given the full sweep intended by the Legislature.” Gilet v. City Clerk of Lowell, 306 Mass. 170, 175 (1940).
The referendum gives the voters a veto power over measures to which it applies. Moore v. School Comm. of Newton, 375 Mass. 443, 446-447 (1978); Morra v. City Clerk of New Bedford, 340 Mass. 240, 242 (1960). Referendum does not permit voters “to bring about something that the city council itself could not accomplish directly.” See Gilet v. City Clerk of *730Lowell, supra. Cf. Sparta v. Spillane, 125 N.J. Super. 519, 527 (1973). Nothing in the referendum process detracts from the notice, hearing, and other requirements of The Zoning Act. Cf. Sparta v. Spillane, supra; Hancock v. Rouse, 437 S.W.2d 1, 4 (Tex. Civ. App. 1969).
The Zoning Act contains no explicit or implied exception to the referendum statute, and the referendum is neither repugnant to nor inconsistent with the purposes of The Zoning Act. We will not imply an exception to it unless such an exception is clearly intended. See Boston v. Board of Educ., 392 Mass. 788, 792-796 (1984); Cohen v. Price, supra. We therefore conclude that amendments to Haverhill’s zoning ordinance are subject to referendum.
2. Retroactivity of the referendum. Lane also argues that, even if the referendum applies to zoning ordinances, it applies only as of the date the referendum petition is filed, but does not operate to defeat rights created before the petition is filed.7 *731Lane points to the language of the referendum statute, which provides that, when a referendum petition is filed, the measure “shall thereupon and thereby be suspended from taking effect.” If the measure is not “entirely rescinded” by the city council, it is to be submitted to the voters in a referendum. If it is not approved by a majority of the voters, it “shall forthwith become null and void.” G. L. c. 43, § 42. Lane contends that this language establishes that the referendum does not nullify the zoning amendment retroactively to the date of passage, but only as of the date of the filing of the referendum petition. Lane contrasts the phrase “entirely rescinded,” used with respect to the council’s action, with the phrase “forthwith become null and void.” Lane contends that the use of these two different formulations in the same sentence indicates an intended distinction; that the council’s action rescinds the measure as of the date of enactment, but that the voters’ power through the referendum does not work retroactively. The special permit, approved by the city council on September 9, 1986, would therefore not be affected by the referendum petition filed on September 15, 1986, because the amendment on which the permit was based was valid at the time and was only invalidated later, and prospectively. We do not agree.
We have held (under former G. L. c. 40A, § 7) that a veto by a mayor renders a zoning ordinance “nugatory.” Kubic v. Chicopee, 353 Mass. 514, 516 (1968). Under G. L. c. 40A, § 5, when the Attorney General disapproves a town zoning by-law, “the previous zoning by-law . . . shall be deemed to have been in effect from the date of such vote.” The same principle applies here: The referendum voided the zoning amendment. Generally, “forthwith . . . null and void” means *732immediately null and void. However, we read a statute as a whole, Commonwealth v. Lamb, 365 Mass. 265, 268-269 (1974), so as to accomplish the purpose for which it was enacted. Selectmen of Topsfield v. State Racing Comm’n, 324 Mass. 309, 312-314 (1949). Read as a whole, the statute shows a legislative intent that the referendum vote rescinds the ordinance as of the date of enactment. As the Superior Court judge noted: “[T]he statute’s explicit reference to rescission makes clear the defect in [the] defendant’s argument. The city council may either rescind the measure or submit it to the electorate with the implicit understanding that they [may] rescind it, i.e., render it null and void [from the date of enactment].” Any other construction would be inconsistent with the legislative purpose in permitting the referendum.
The referendum rescinded the amendment to the zoning ordinance. Because the special permit was expressly conditioned on the amendment, it cannot survive the defeat of the amendment. The Superior Court judge therefore correctly declared that the amendment was subject to the referendum and the special permit was null and void.
Judgment affirmed.