Labor's energy spokesman Mark Butler has refused to back the party's former emissions target, saying it will take years to firm up a new climate change policy while repeating his calls for an "unsparing, ruthless" review of the Opposition's failed election campaign.

Mark Butler has refused to back Labor's former 45 per cent emissions reduction target. Credit:Alex Ellinghausen

Mr Butler said the new medium-target would be "consistent with the best scientific advice" about how to meet the commitments of the Paris Agreement and keep global warming "well below 2 degrees", saying the government's 26 to 28 per cent target was "grossly inadequate".

He would not commit to Bill Shorten’s 2030 target of 45 per cent after former deputy prime minister Tanya Plibersek said during the week that she still supports an "ambitious" target, but said Labor wanted "net zero emissions by the middle of the century".

"Over the course of the period between now and the 2022 election, we will make sure that we have a policy that is also informed by the best advice available to us at the time," Mr Butler told the ABC's Insiders program on Sunday.