Senator Ricky Muir says he couldn't ignore a letter from asylum seekers in detention on Christmas Island desperate to get out.
Ricky Muir has described his anguish at speaking with case workers on Christmas Island who knew his vote as a crossbench senator would determine the future of so many asylum seekers in detention.
Senator Muir, who has confirmed he will support the federal government's overhaul of migration laws, said coming to a decision on the controversial bill was one of the hardest decisions he'd ever faced.
The government needs the support of six crossbench senators to get its legislation through the Senate, and all but Senator Muir had confirmed on the record they'd back the changes.
The bill, among other things, will reintroduce temporary protection visas for refugees and increase powers for turning back asylum seeker boats.
As the legislation was being debated on Thursday, Senator Muir took a call from staff working with asylum seekers at Christmas Island who promised to speak with the politician whose vote counted.
"They told the people in detention that they rang the office of the man whose decision it was to decide whether they would be out of detention before Christmas," he told the upper house.
"That man wasn't the minister for immigration. It was me.
"It shouldn't be like this, but it is. The crossbench shouldn't have been put in this position, but we have."
The Motoring Enthusiast Party senator, at times emotional, said he could not have voted for the bill in its original form and there were aspects he wasn't comfortable with.
But he feared that doing nothing wouldn't help the 30,000 people languishing in detention centres across Australia.
He believed the amendments put forward by the Palmer United Party and independent senator Nick Xenophon could potentially help those refugees in limbo find a way out.
But it wasn't an easy call for the senator, who has wrestled with some tough questions on the crossbench since entering the chamber in July.
"I am forced into a corner to decide between a bad decision and a worse decision," he said, "a position I do not wish on my worst enemies."
Debate on the bill is continuing in the Senate.