Prime Minister Tony Abbott faces parliamentary dissent on a new front after two senior Liberal senators signalled they will not support the government's new deficit levy in the Senate.

In an impassioned speech to the Senate on Monday, Senator Ian Macdonald said he would oppose the federal government's plan to introduce a 2 per cent tax on incomes of $180,000 or more, saying he did not think it went far enough.

Senator Macdonald, who was demoted from the frontbench after the election, said he did not understand why the levy did not apply to companies as well as individuals.

He later told Fairfax Media he would ''probably'' cross the floor, but would make up his mind after the debate was completed in the Senate.

He was soon joined by fellow Liberal Senator Cory Bernardi, who said he said he was ''unable to support'' the levy, arguing that the government should be ''looking to lower taxes in this country''.

A spokeswoman for Senator Bernardi said he would abstain from voting rather than cross the floor.

Both senators acknowledged that their votes would not impact on the outcome of the vote. With Labor planning to vote in support of the levy, it will easily pass the upper house when it goes to a final vote, which is expected on Tuesday.

The debt levy dissent comes as Senator Macdonald said he might also cross the floor over Mr Abbott's ''signature'' paid parental leave scheme. He told the Senate he was seeking answers on how the proposal would affect voters in his state and stay-at-home mothers.

Senator Macdonald has previously been included in a group of Coalition MPs unhappy with the paid parental leave scheme that includes Nationals John Williams and Ron Boswell and Senator Bernardi.

Labor seized on Senator Macdonald's comments on Monday, with Opposition Leader Bill Shorten asking Mr Abbott in question time: ''If the Prime Minister's own colleagues won't even support this unfair paid parental leave scheme, why should Australia?''

Mr Abbott stood by his policy, arguing it was an ''important social and economic reform and it will be delivered by this government''.

The Prime Minister's signature policy has not yet gone to cabinet but it is understood it will be presented in the coming weeks.

Liberal backbenchers who support the scheme have written to Mr Abbott with a proposal that would see it implemented on a trial basis for five years. That trial would measure the impact on productivity and workforce participation, two of the reasons Mr Abbott cites in support of the policy.

Sources close the Prime Minister say he believes he must present his policy in unamended form at least once to the Parliament, though it could be defeated.

Mr Abbott has already changed the policy, lowering the income cap from $150,000 to $100,000 in April, bringing it closer in line to Greens.

with Lisa Cox

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Correction: An earlier version of this story said the income cap for the paid parental leave scheme was lowered to $50,000.