Scott Morrison has again sternly warned Australians against travelling to Ukraine to take up arms as Russia's invasion continues.
The prime minister said the legal position remained unclear at best with Australians prohibited from becoming foreign fighters.
But he admitted it would be hard for authorities to stop Australians leaving the country and stopping over in third countries before travelling to Ukraine.
"People are allowed to leave Australia. If they are going there I doubt they would be on a direct flight to Ukraine," he said.
"They may go to a number of places. So without that clear intent, and without any intelligence, I think it would be an overestimation as to how Border Force could intercept on those occasions."
Labor said it would support the government making it a criminal offence to enter or stay in designated Ukrainian regions to prevent Australians joining Russian forces.
The party's home affairs spokeswoman Kristina Keneally said a small number of Australians had previously travelled to Ukraine to join Russian-backed militias.
"Some Australians, small in number but extremist in their view, are spreading Russian propaganda and encouraging people to consider going to support the Russian forces," the senator told Sky News.
"We shouldn't allow people to travel, to train and work with those who seek to do us - or indeed our strategic partners - harm."
Mr Morrison didn't directly respond to the proposal, instead reiterating calls not to travel to Ukraine.
"What I'd also argue is anyone seeking to do that would find themselves on the wrong side of some very, very violent attacks," he said.
"Those disorganised civilian militias would effectively be putting people in the most extreme of situations so we are certainly not encouraging anybody to go there."
He said it remains unknown how any foreign fighter would support Ukraine's military forces or fit into any command structure.
The head of the Ukrainian embassy in Canberra said it was important that Australians complied with the legislation, after about 20 people had contacted the embassy on Monday morning to inquire about heading overseas to fight.
"I feel really pleased to hear such words of support. At the same time, it's important to carefully evaluate how you can proceed," Charge d"Affaires Volodymyr Shalkivskyi said.
"We appreciate such willingness from Australian people to help Ukraine to defend our country (but) there are different ways to support our military and to protect our civilians (like) financial assistance, humanitarian aid."
Defence Minister Peter Dutton warned the "slaughter" of men, women and children in Ukraine by Russian soldiers will increase as forces move closer to the capital, Kyiv.
"Russian forces are frustrated by the resistance on the ground and have underestimated the strength of that resistance," he said.
"As the Russians ramp up their activities, we know from history that Russian troops can be particularly brutal and people should brace themselves for what we will see out of the Ukraine in the coming days and weeks."
The federal government has provided lethal military aid including missiles and ammunition as part of a $105 million package of support which also included humanitarian relief.
