Opposition Leader Malcolm Turnbull and Coalition immigration spokeswoman Sharman Stone both argued against the Bill in Tuesday's joint party room meeting.

But Mr Georgiou and Ms Moylan, who have both criticised the party in the past for their approach to asylum-seekers, have supported the Bill, while fellow moderate Russell Broadbent could also support it.

Mr Georgiou said former Keating government immigration minister Gerry Hand had established mandatory detention in 1992 through a combination of ''wickedness, manipulation and attacks on churches and do-gooders''.

The Howard government had, to its ''shame'', pushed through tougher laws, including establishing temporary protection visas for legitimate asylum-seekers.

But the reality of mandatory detention was different, he said, as it kept families apart.

''In the five years prior to the introduction of TPVs, there were 3103 boats arrivals. In the five years after TPVs were introduced, there were 11,433 arrivals,'' he said

Mr Georgiou said the law that charged asylum-seekers for their incarceration at the rate of $125 a day had ''utterly failed''.

''Since the policy was initiated, only 4 per cent of the costs have been recovered. Over the last four years, $139million or 81.7 per cent of charges have been waived or written off, mainly by the Coalition government, because it was impractical or uneconomical to recover them,'' he said.

Ms Moylan said it was important to remember the new law did not cover people smugglers or illegal fishermen found in Australian waters, who would still face sanction.

''This law was never intended to deter people from coming to Australia ... a few boatloads of people cannot be construed as a threat to national security, despite the hysterical headlines that greet every boat arrival,'' she said.