'Our idea': Treasury says it came up with tax cut changes

But Treasury officials told a Senate committee hearing that the government did not instruct the department to include tax changes in the proposals it was developing.

The Opposition, which has claimed the government's stage three tax changes are a "betrayal" of voters, grilled Treasury officials on Monday on when and how the idea to overhaul the tax cuts came about.

Head of Treasury's revenue group, Diane Brown, told the Senate cost of living inquiry that the department was asked to begin developing options for cost of living relief following a meeting between Treasurer Jim Chalmers and Treasury secretary Steven Kennedy on December 11 last year.

In public statements following that date, both Prime Minister Anthony Albanese and Dr Chalmers continued to affirm Labor's commitment to the stage three tax cuts legislated by the previous government.

Opposition finance spokeswoman Jane Hume and her WA colleague Dean Smith, who accuse the government of lying to the public, pressed Treasury officials on whether the treasurer had instructed the department to consider changes to the tax cuts even while publicly affirming his commitment to them.

But Ms Brown said the department was "always looking" at the tax system had taken it upon itself to include changes to the stage three cuts as part of options to be considered in providing relief for households under financial strain.

She said following his conversation with the Treasurer on December 11, Dr Kennedy asked officials to develop options for further living cost relief and the only parameter imposed was that such assistance would not be inflationary.

"The [Treasury] secretary was very concerned that any cost of living relief didn't add to inflation," Ms Brown said.

"Preliminary work began on a range of options but, in the end, given our objective of not having cost of living relief that added to inflation, we settled on the advice that was provided [to Dr Chalmers] on January 20."

Asked whether the government ruled out changes to the tax system in asking for Treasury's advice, Ms Brown said she was not involved in the conversation between the treasurer and Dr Chalmers in December.

"[But] the impression I got was that it was made clear that the Treasurer did not necessarily agree with changing stage three," she said.

"That it was the government's commitment to retain stage three as legislated but that if Treasury wanted to do work on it they could."

Treasury's advice on the tax cut changes was delivered to Dr Chalmers on January 20 and was considered by the powerful Expenditure Review Committee on January 23 before being presented to Cabinet the following day.

Mr Albanese formally announced the tax changes on January 25.