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And while Mr Butler has said Labor was "still taking advice" on whether Kyoto credits could be used, his spokeswoman told the Herald on Thursday, the ALP had yet to ask for a briefing.

"We’ll seek advice from [the Department of Foreign Affairs and Trade], the Department of Environment and any other relevant agencies," she said.

The Herald asked Environment Minister Melissa Price and her department what evidence they had that indicated Kyoto credits were eligible for use after 2020.

Ms Price said the Paris Agreement and Kyoto Protocol were separate agreements: "Targets under the Paris Agreement, and how countries go about meeting them, are nationally determined."

A department spokeswoman said "the Australian Government does not generally comment on or disclose its privileged legal advice".

But Malte Meinshausen, a former European Union negotiator and head of Melbourne University's Climate and Energy College, said the use of at least the first Kyoto period credits - from 2008-2012 and totalling about 128 million tonnes for Australia - was clearly not permitted.

'Cook the books'

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The Herald understands the Greens are exploring if an amendment can be moved to any bills to prevent the government from using surplus Kyoto credits. They would seek Labor's support to make it law.

“We expect the conservatives to cook the books on Paris, but Labor shouldn’t follow their lead," Adam Bandt, the Greens' climate spokesman, said. "Labor must commit to cancelling any surplus credits if it forms government."

Dave Sharma, the Liberal candidate for the Sydney seat of Wentworth, said the use of such permits was acceptable. Earlier this week, he said Australia "should be doing more".

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“It’s a strange assertion that if you’ve out-performed your targets before, you should be penalised," he said. “I don’t think it lessens our credibility at all.”

Kerryn Phelps, who beat Mr Sharma to win the Wentworth seat last year, said the government was "gleefully" using an accounting ruse to avoid proper emissions goals as set out in the Paris accord.

"The point of the Paris Agreement is to ratchet up our emissions targets over time, not for the government to use weasel words to water them down," Dr Phelps said.

"Australia should be showing leadership on the international stage not moving the goalposts so we can avoid our international responsibilities."

With Michael Koziol