The Prime Minister's paid parental leave scheme has received little attention in the federal budget papers.

Tony Abbott recently pared his signature scheme back so the maximum payment for six months would be $50,000, but the policy only appeared in one paragraph in Tuesday night's budget.

Provision has been made for it in the contingency reserve, a bucket of money reserved for decisions taken but as yet not announced by government, decisions too late to be included in portfolio estimates and so on.

Grattan Institute chief executive John Daley spotted it last night.

"You need a scanning electron microscope to find it in the budget papers," he said.

"It gets a very brief mention in the revenue section, indicating that the company tax cut and company tax increase have already built into the numbers as a parameter and other variation, apparently last November.

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"But without any clear indication of exactly how much is there, and then on the cost side, the spending side for paid parental leave, we're told that that's already built into the contingency reserve ... we're told that's changed a little bit as a result of the change in threshold, but not how much.

"And that is the only thing that you will find in terms of references to paid parental leave - apart from a single line in the Treasurer's speech - in literally hundreds and hundreds of Treasury documents about the budget.

"So it is unusually short treatment for something which is apparently a signature policy of the Government."

The official Treasury explanation is the Commonwealth is still negotiating with the states about their contribution to the scheme, and the funding is included in the budget's contingency reserves.

The same response has come from Treasurer Joe Hockey.

"We are still negotiating with the states about the scheme and, as you know, we've reduced the threshold from $150,000 to $100,000 in relation to the PPL," he said.

"I notice that there weren't many journalists criticising us for breaking that promise."

Mr Daley says that might explain its formal classification.

"Although I would note that there are lots of other things about which there are uncertainties which, nevertheless, go into the budget, particularly at the point that they are formally government policy," he said.

"What that doesn't explain is why there is so little airplay for an important policy. Even if there are plenty of uncertainties around it, it's already in the numbers in effect."

Absence 'reflects ambivalence' to policy

Mr Daley says it may be due to a lack of party unity around the policy.

"I think it may well reflect there is a certain amount of ambivalence about it as a policy. I think it's a matter of public record that not all in the Coalition are entirely convinced that it is sound policy.

"The Commission of Audit obviously recommended that it be cut back even further, and I think picking up very sound international evidence that we also looked at the Grattan Institute that showed that for every dollar you spend on paid parental leave, you would get double the impact on female workforce participation if you spent it subsidising child care.

"So there are really good reasons why, in a tight budgetary environment, this is not the right policy and I think the relatively short treatment may reflect that not everyone is entirely united behind it."

Last election, the Greens backed the same payment level for parental leave that Mr Abbott has now agreed to, but the minor party is yet to say if it will pass it in the Senate, demanding the Government first provide all the financial details and that the scheme be fully funded by big business.

Now the budget has been handed down, Greens MP Adam Bandt is stressing not one cent of the scheme be paid for by spending cuts and says he is disappointed that no more details have been laid out in the budget papers.

"It's disappointing that there's still not a clear, fully costed policy on the table explaining how the proposed paid parental leave scheme would work," he said.

"That's because Tony Abbott still doesn't have the full support of his Cabinet or his caucus.

"Perhaps it's because they haven't finalised the details, but the full details of the proposal are going to be crucial to know. For our part, we'll not accept any proposal that is funded by even one dollar of the horrendous cuts that we're seeing in this budget.

"At the moment, we haven't even got to square one because we haven't yet seen a detailed policy that the Prime Minister can say with his hand on his heart has the support of his side of politics.

"There's no point in wasting the public and the Parliament's time if there's a scheme that's going to be put up that is going to see Liberal members of Parliament refuse to vote for it."