A heated dispute has broken out over Australia's next key climate target after revelations the federal government might have been working on the figure for at least a year.

Treasury has been modelling the economic impacts of plans to cut greenhouse gas emissions by 2035, which are widely expected to be unveiled this week.

The department has been working on the target since "early last year," officials told a Senate inquiry on Tuesday, meaning work was under way well before the federal election in May.

Greens leader Larissa Waters said it was an "explosive revelation".

The department's admission made "a mockery of the process of actually looking at the climate science and determining the risks", she told reporters in Canberra.

Treasury revealed it was asked to model a single figure for

See Full Page