Treasury's credibility is on the line.

The Government's economic advisors have admitted to a coding error that has a big impact on the impact on the Government's Families Package.

Treasury had previously projected that 88,000 fewer children would be in poverty by 2021, but now they are unable to stand by those numbers.

Treasury Secretary Gabriel Makhlouf said it's a deeply regrettable mistake.

It will be causing a headache for the Government as its major policy promise has been to reduce the number of children in poverty.

READ MORE: Treasury admits blunder over child poverty

Victoria University's Dr Bryce Edwards said questions will be asked whether Treasury is competent in doing its job.

"There will be some people questioning how robust processes are at treasury if they got this so wrong. This was the main issue in the 2017 general election."

Public submissions called by Parliament on the issue are meant to close tomorrow.

Dr Edwards said people can't be expected to make meaningful submissions if they don't have the correct child poverty numbers.

"Treasury now say that they won't have the correct child poverty reduction numbers until the end of February, and really it makes a mockery of parliamentary submission process."