The Morrison government's plan to rely on surplus "credits" to meet more than half the Paris climate goal is a "huge gamble" that could leave a funding gap if other nations formally rule out their use, according to a former Australian climate negotiator.

Richie Merzian, director of The Australia Institute's climate and energy program, said the Coalition's intention to count the expected surplus of 367 million tonnes of so-called "carryover credits" generated by beating the Kyoto Protocol climate target to 2020 may be ruled unacceptable by other nations.

The Coalition's climate policy relies heavily on an accounting "surplus" that could leave a large, costly hole if the rules for Paris block it, The Australia Institute says. Credit:Andrew Meares

Mr Merzian noted the Paris climate accord signed in 2015 for 2021-30 specifically "encourages parties to promote the voluntary cancellation" of credits during the Kyoto's first and second period. Other nations, such as Germany, New Zealand, Sweden and the United Kingdom, had all cancelled anticipated surpluses.

"There is a question as to whether Australia will even be able to carry over all of its surplus," Mr Merzian said in an analysis of the credits.