British Prime Minister Margaret Thatcher is suggesting Australian Prime Minister Malcolm Fraser as a possible moderator of the Zimbabwe-Rhodesia dispute.

Thatcher, visiting the Australian capital of Canberra after attending the Tokyo summit meeting, was warmly welcomed by Fraser, who is known to be eager for the task.

Details of the Thatcher proposal were not immediately made public, but officials indicated that Thatcher wanted Fraser to take a leading role at the meeting of Commonwealth heads of government in Lusaka, Zambia, in August.

For an Australian conservative, a millionaire who owns an 8,000-acre cattle ranch, Fraser has unusual qualifications to be a moderator in the dispute over the future of Zimbabwe-Rhodesia.

All his predecessors as leader of the Australian Liberal Party- the de facto conservative party-were more inclined to support the policies of the white minority government of former prime minister Ian Smith in Rhodesia and the white minority governments in South Africa than the black African position.

But Fraser, who has been in office 3 1/2 years, came out strongly in support of majority rule in Rhodesia at his first Commonwealth meeting in London two years age. Despite strong, continuing opposition from conservatives in his government, Fraser has since referred to the country only by its African name: Zimbabwe.

One reason Thatcher apparently has chosen Fraser for a peacemaking role in Lusaka is that Fraser's views of the white-supported government of Bishop Abel Muzorewa are known to be acceptable to the black African nations. She also knows that those nations are suspicious that Thatcher will both recognize the Muzorewa government and lift British sanctions imposed when Rhodesia unilaterally declared its independence in 1965.

Fraser and his foreign minister Andrew Peacock, have made it clear that Australia will not rush to recognize Muzorewa. They made that clear to Andrew Young, the U.S. ambassador to the United Nations, when he was in Canberra shortly after the Zimbabwe-Rhodesia elections in May.

The recent raids on suburds of Lusaka, Zambia, apparently by Zimbabwe-Rhodesian security forces, stiffened Fraser's determination not to recognize the Murzorewa government and to maintain Australian sanctions against it.

Thatcher, meanwhile, strongly criticized the Vietnamese government concerning the refugee problem.

"Vietnam is a callous, cold, cruel dictatorship," she said in an Australia television interview. "We must never forget the reason for the refugees' misery. The reason is that this is a Communist government in action."