Nationals back 'flexible' net zero pledge

"It leaves us wiggle room as we proceed into the future. Morrison's statement that he has made is not binding. There will be no legislation attached to it," Mr Boyce told the ABC.

The Liberals and Nationals agreed to a target of net zero emissions by 2050, following tense negotiations ahead of the UN climate change conference COP26 in late 2021, but the target will not be legislated.

However, moderate Liberal MP Dave Sharma - who is facing down a challenge from "teal" independent Allegra Spender - has described the commitment as "binding". 

Mr Joyce told AAP that Australia needs to continue exporting coal and backed his candidate after he talked down the government's net zero emissions target.

"We've said we've set a target, we're going to try and meet ... but I think where Colin's coming from, it's completely understandable," he said. 

"Coal remains one of our nation's strongest exports.

"If you start getting rid of one of our major exports, then our key economic strength is affected."

Mr Boyce says the document allows for flexibility under clauses such as "the future of gas and coal industry will be determined by the pace of international change and the demand from our international customers".

In an interview with Sydney publication the Wentworth Courier two weeks ago, Mr Sharma is quoted as saying that he changed the party's climate policy.

"I've delivered a binding commitment to net zero emissions by 2050, an upgraded 2030 emissions reduction target and obtained a guarantee that the offshore oil and gas project pep-11 will not proceed," Mr Sharma was quoted as saying.

Meanwhile, Labor's climate change and energy spokesman Chris Bowen questioned whether the coalition "says one thing in Collinsville, another thing in Kooyong".

"The LNP candidate for Flynn says the Morrison Govt's net zero by 2050 target is "flexible", "not binding" and has "wriggle room". Do you agree Josh Frydenberg?" he tweeted at the treasurer following the comments.

Shadow treasurer Jim Chalmers was also forced to clarify Labor's position on coal after a stumble by deputy leader Richard Marles.

Mr Marles on Saturday appeared stumped when asked about how coal fit into the party's emissions reduction policies. 

He also repeatedly avoided answering whether coal mines would be forced to buy carbon credits after a certain amount of emissions. 

On Monday, Mr Chalmers said Labor's climate policy "is not about abandoning our traditional strengths in areas like mining" - a line he also used to jump in to answer the question directed at his deputy leader on Saturday.

Mr Chalmers defended the intervention, rejecting that Mr Marles was stumped by his party's position towards coal.

"In these press conferences, what typically happens is whoever's at the microphone, they throw to whoever is best place to answer the question," he told the Nine Network. 

"I usually put my hand up to answer the climate change questions if Chris Bowen is not there, because it's an area of policy passion and interest for me, and that's what happened.

"I answered it fully and completely. I don't know what the fuss is about."

Mr Bowen confirmed that the party's policy to have 215 emitters reduce their emissions to net zero by 2050 included coal mines.