AUSTRALIA is taking aim at the buyers of semiautomatic military- style assault rifles.Most of Australia's state governments are now moving to ban the sale of such weapons. New South Wales (NSW), Tasmania, and the Australian Capital Territory (ACT) are now tightening up the laws on ownership of semiautomatic rifles and shotguns. Victoria, Western Australia, and the Northern Territory already have tougher laws making ownership of assault rifles very difficult. The tighter state laws will get a national airing on Oct. 23, when the police ministers from Australia's six states and two territories meet to discuss gun-control measures. Then, in November, the state premiers will discuss the issue with Prime Minister Bob Hawke, who has always maintained gun control is up to the states. Mr. Hawke, however, says the Commonwealth is now reviewing the existing laws on import restrictions. The government prohibited the importation of all military-style guns more powerful than .22 caliber in December 1990. A catalyst for the change was the massacre of seven people and wounding of six others in a shopping center in Strathfield, NSW, on Aug. 18. The assailant, who killed for no apparent reason, used a Chinese-made SKS assault rifle. Several weeks earlier world-famous heart surgeon Victor Chang was gunned down on the streets of Mosman, a wealthy Sydney suburb. Dr. Chang's daughter appealed for gun-control legislation. The killings galvanized the leaders and public opinion. While in recent NSW elections, opposition leader Bob Carr and Premier Nick Greiner both sidestepped the gun-control issue - which had derailed a Labor politician, Barry Unsworth, two years ago - both agreed Tuesday on a bipartisan package of gun-control laws. Among its elements are: the suspension of new shooters licenses until Jan. 1, 1992; the requirement that all applicants for shooting licenses complete a safety course; the suspension of the sale, resale, and purchase of all semiautomatic rifles and shotguns until Jan. 1, 1992 "with a view toward a permanent ban thereafter"; and amnesty where illegal guns can be turned in until the end of the year. It wasn't hard for the politicians to gauge public opinion. A Rupert Murdoch-owned newspaper, the Telegraph Mirror, asked readers to sign an anti-gun petition. It collected 1 million signatures. The gun-control movement also picked up the support of the unions. On Tuesday, many union members participated in a march to Parliament House. According to Geoff Derrick, assistant secretary of the Finance Sector Union of Australia, over 10,000 of its members have been the victims of holdups over the past five years. "It's a risky environment," says Ross Pitman, an employee of Westpac, the nation's largest bank. The rally was the idea of John Hennessey, deputy mayor of Campbelltown, a Sydney suburb. "I just said enough is enough and the idea snowballed," says Mr. Hennessey. Other politicians joined the rally. "We have to put a stop to the massacres," says Dawn Fraser, former member of the NSW Parliament. The rally attracted many middle-class Australians. ve never done anything like this," says Mrs. Jenkins from Carlingford, NSW. "But, we've got to do something before we're like America." The gun lobbies have promised marches and an advertising campaign in the next few weeks to fight the legislation. It is estimated that Australians own 3.5 million guns, or one gun for every five people. Many of the guns are owned by sportsman who belong to gun clubs and ranchers who use them on their stations.