Liberal MP Russell Broadbent says he won't vote with his coalition mates to change the Migration Act because he doesn't believe in offshore processing.
Despite the coalition's policy to reopen Nauru, Mr Broadbent said he was in favour of processing asylum seekers onshore only - a position he said hasn't changed since 2005.
"I am in favour of onshore processing (because) it costs a boatload of money to have people housed offshore," he told ABC Television on Thursday.
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"All of that money could be spent on secondary schools, country hospitals - all of those things are really important to us."
The government is contemplating changing the law to allow it to keep offshore processing as an option, which would also keep alive its scuttled Malaysia people-swap proposal.
But asked if he would support any amendments to the Migration Act, Mr Broadbent said no.
"Look, I can't see myself voting for that because I don't think it's practical for the nation now," he said.
"I don't see that the High Court would allow it in any way whatever either party says at the moment.
"There are other parts of the (United Nations refugee) convention that I would see that overseas processing would not allow for."
Mr Broadbent, the member for the Victorian seat of McMillan, said Opposition Leader Tony Abbott and the Liberal Party were well aware of his position, having voted against the Nauru detention centre in the past, along with fellow Liberal MPs Judi Moylan and Petro Georgiou.
He played down comments from the immigration department that Australia faces civil unrest on the same scale as London and Paris should it turn to onshore processing only.
The department is worried that processing on the mainland would see a flood of 600 people arriving each month, bringing Australia's detention network to bursting point within a year.
The comments prompted Australian Greens leader Bob Brown to label the immigration advisers "turkeys" and "xenophobic", saying they should be sacked.
Mr Broadbent said there was a way to make onshore processing work.
"Let's not blame public servants," he said.
"I think the civil unrest (department secretary Andrew Metcalfe) was talking about was if the detention system doesn't change.
"I do think there's another way (to change the detention system) if we're prepared to do it as a nation."
