A promise by Malcolm Turnbull to the Nationals to keep the baby bonus will cost $1.4 billion over the next decade.
The prime minister pledged to retain the bonus in the wake of him ousting Tony Abbott and settling a new coalition agreement with the Nationals in September.
From July 2016, all eligible families with a youngest child under one year are to receive an extra $1000 a year through an increase to their FTB-B standard rate.
An analysis by the Parliamentary Budget Office, obtained by AAP, found it would cost $1.4 billion over the next 10 years.
Labor families spokeswoman Jenny Macklin said Mr Turnbull's priorities were wrong, as at the same time the government was cutting family benefits by as much as $5000 a year.
"So much for a budget emergency, so much for simplifying the welfare system, and so much for putting good policy over political expediency," Ms Macklin told AAP on Tuesday.
Opposition finance spokesman Tony Burke said the prime minister had chosen political expediency over good policy.
Labor has pledged to reverse the measure if elected to government.
"This is an example of Labor taking a sensible approach to reducing expenditure in a responsible way," Mr Burke said.
