Earlier, Treasurer Josh Frydenberg had defended using Treasury to cost Labor's tax proposals saying voters need to know the choice they face at the May 18 federal election.

"These are Treasury costings and, as Wayne Swan said when he was treasurer of Australia, Treasury do cost alternative policies and it's important to have an informed debate," Mr Frydenberg told reporters in Melbourne on Friday.

Prime Minister Scott Morrison said this was a process Treasury has done for a long time for parties on both sides of politics.

"The real question is why hasn't Bill Shorten told you about how much more his taxes are going to cost you?" he told reporters in Sydney.

But Mr Gaetjens has told Mr Bowen in a letter that Treasury completed costings based on details and specifications submitted by Mr Frydenberg's office, not on Labor's policies.

"We were not asked to cost another party's policies and would not do so," the Treasury boss said in a response to a letter from Mr Bowen.

Mr Frydenberg said the costings showed Labor's tax plan would cost an extra $387 billion over the next decade, almost double previously estimated - a total Mr Gaetjens said his department had not provided.

Mr Bowen said Mr Gaetjens letter showed Mr Morrison and his treasurer had again been caught out lying about Labor.

"This is a humiliating rebuke and confirms that Scott Morrison cannot be trusted on the economy," Mr Bowen said in a statement.

"Every time Scott Morrison talks about Labor and the economy for the next five weeks, Australians will know that Scott Morrison is lying to them."

The vast majority of the costings - $230 billion - comes from Labor's decision not to match the coalition's personal income tax cuts in 2022 and 2024.

"Bill Shorten's taxes not only stifle aspiration but they will also hurt the economy, costing jobs and putting at risk the essential services Australians rely on," Mr Frydenberg said.

Mr Shorten said this was a desperate government that is willing to "lie about anything".

"They're now saying that because we won't agree to something they haven't done in six years' time, that this is a tax increase on Australians. This is a fairytale," Mr Shorten told reporters in Sydney.

Treasury's costings predicted ending taxpayer-funded handouts for franking credit refunds - a policy Labor would pursue in government - will raise an extra $57 billion, and $34 billion from changes to superannuation thresholds.

A further $31 billion would come from ending negative gearing on existing properties and changes to capital gains tax, and $27 billion from changes to family trusts.

Australian Associated Press