Representatives of 41 British Commonwealth nations today called on the rest of the world to support urgent new talks between rich and poor countries to deal with worsening global economic disparities.

The appeal came in a joint declaration by leaders of the Commonwealth, which groups Britain and its former colonies. Issued in the Australian capital of Canberra during the leaders' weekend retreat, the document, called the "Melbourne Declaration," asked "real and significant changes commensurate with the urgency of the problems we now face."

The declaration mentioned no specific measures, however, and a controversy erupted when one Commonwealth leader, New Zealand Prime Minister Robert Muldoon, called it a "declaration of platitudes."

Commonwealth officials said the document was a "political statement" and that specific economic issues such as food, finance, trade and energy would be addressed in the final communique of the Commonwealth conference this week.

In a separate statement, Australian Prime Minister Malcolm Fraser made it clear that the declaration was intended to give impetus to this month's North-South summit at Cancun, Mexico. That meeting, to be attended by 22 world leaders including President Reagan, is to discuss economic issues dividing rich and poor nations.

In the Melbourne Declaration, the Commonwealth leaders said it was "imperative to revitalize the dialogue between developed and developing countries." It said the world's "gross inequality of wealth and opportunity" and an "unbroken circle of poverty" in developing countries constituted "fundamental sources of tension and instability in the world."

The statement said the leaders "firmly believe that the choice is not between change and no change but between timely, adequate managed change and disruptive, involuntary change imposed by breakdown and conflict."

Third World countries, which make up the vast majority in the Commonwealth, hope the Oct. 22-23 summit in Cancun will clear the way for a new round of North-South talks on creating what they call "a new international economic order."

Begun in Paris six years ago, the North-South talks between representatives of Western industrialized countries and developing nations soon bogged down and were eventually broken off. A special session of the United Nations last year failed to agree on an agenda and procedures for another round of talks, and Third World countries blamed the United States, Britain and West Germany for the deadlock. The developing countries basically seek a redistribution of the world's wealth.

Although a conservative on Australia's domestic political scene, Fraser has been leading the campaign for new North-South talks.

In his speech today he said that "the protracted assault on human dignity and the deprivation from which many millions in developing countries suffer must inevitably lead to political turmoil." Such turmoil, he said, would be used "to extend the realm of dictatorship in the world."

However, the Melbourne Declaration was sharply criticized by Muldoon, who told a news conference it was released prematurely. He said he wanted the declaration to contain a strong condemnation of protectionism in international trade.

"I support what is in it," he said. "I just do not think it is more than a pious declaration composed principally of platitudes."