KUALA LUMPUR, Malaysia (AP) _ A court today convicted and sentenced to death an Australian man on charges of heroin trafficking, rejecting his claim that he thought the powder seized in his 1985 arrest was an aphrodisiac.

The death sentence could again strain relations with Sydney. When two Australians were hanged in Malaysia on drug charges in 1986, Australian Prime Minister Bob Hawke called it a ''barbaric act.''

Michael Dennis McAuliffe, a 35-year-old former bartender from Brisbane, was found guilty of possession of five ounces of heroin stemming from the June 1985 arrest at Penang International Airport.

McAuliffe appeared calm when he spoke to reporters after the verdict.

McAuliffe said he was satisfied that his lawyer, Karpal Singh, ''has done his best.''

Singh said the sentence, issued in the high court at Penang, 180 miles north of the capital of Kuala Lumpur, would be appealed to the Supreme Court.

McAuliffe is the third Australian to be sentenced to death in Malaysia under a stringent drug law that carries a mandatory death sentence for convicted drug traffickers.

The two other Australians, Kevin John Barlow and Brian Geoffrey Chambers, were sentenced in 1984 and hanged July 7, 1986. A third Australian detained on drug charges was released Feb. 18 when prosecutors dropped the case.

In announcing sentence, Judge Abdul Hamid Mohamed today rejected McAuliffe's claims he did not know the white powder he carried was a drug.

McAuliffe had told the court in earlier hearings that a bar hostess in Bangkok introduced him to the substance, which she called ''bang-bang powder.''

McAuliffe said he and a Swiss woman had invited the Thai woman home and she had brought some of the white powder, which she said could be inhaled or rubbed on the body.

He said he bought $350 worth of the substance from her before he left Bangkok. He said he placed it in his money belt because ''while it was not exactly valuable, it was irreplaceable.''

Since 1975, when Parliament allowed capital punishment for drug offenses, 385 people have been sentenced to death. Of those, 110 have been hanged, including Thais, Singaporeans, Hong Kong Chinese, Filipinos and others. The rest are awaiting appeals or execution dates.

Actual sale of a drug need not be proven under Malaysian law. Mere possession of half an ounce or more of a controlled substance is considered trafficking and is punishable by death.

Prime Minister Mahathir Mohamad has defended the stringent laws, saying they are necessary because the drug problem is Malaysia's ''No. 1 enemy.'' Officials estimate that about 150,000 of Malaysia's 18 million people are drug addicts.