The heavyweight contenders in a contest over a proposed hydroelectric power contract that would supply New York State with electricity from Quebec will square off in Manhattan Thursday night.

But this match will not be held at Madison Square Garden. Instead, the Association of the Bar of the City of New York will host a forum at its Great Hall on West 44th St. on the legal and environmental issues raised by the pending power supply agreement between the New York Power Authority and Hydro- Quebec.Speaking at the forum will be Richard M. Flynn, chairman and chief executive of the New York Power Authority; Richard Drouin, chairman and chief executive of Hydro-Quebec; Matthew Coon-Come, grand chief of the Quebec Cree Indians; Jan Beyea, chief scientist, National Audubon Society; and Stephen Blank, director of Canadian Affairs, the Americas Society.

At issue will be a 1,000 megawatt firm power contract between Hydro-Quebec, the giant power producer entirely owned by the province of Quebec, and the Power Authority, a state agency that produces and distributes wholesale electricity.

In 1989, the Power Authority agreed to a 21-year, $17 billion contract to

purchase power from Hydro-Quebec. The Canadian utility expected to produce at least some of that power from its Great Whale hydroelectric project, which was to begin construction this year to meet the start of the contract in 1995.

Advocates of the contract claim it will supply electricity to New York and other Northeastern states at a cost less than it can be produced here. Opponents say the power is unneeded, that the project would be harmful to Quebec's environment and disrupt the native way of life.

The final agreement was to be signed in November, but the Power Authority's Mr. Flynn, citing changing economic conditions, successful conservation measures and the need for environmental review, invoked a contract provision delaying ratification until November 1992.

Then, in September, a Canadian federal court ruled that a full study of the project's environmental impact must be completed before Great Whale's construction can begin. That ruling, endorsed by Mr. Flynn, has indefinitely delayed the start of the project.

Mr. Flynn has said that the environmental study must be well on its way before final ratification can take place, and that in any event the cost of the contract will have to be renegotiated in light of changing economic conditions.

Mr. Drouin has said that if the New York contract is not signed, the Great Whale project will be delayed, but that it will eventually be built.

The public is invited to attend the forum. After presentations by each of the speakers, the chair will entertain questions from the floor.