Richard Di Natale has thanked his Greens colleague Nick McKim for attempting to visit the Manus Island detention centre before being deported back to Australia.
The Greens leader said Senator McKim was a voice of "decency and compassion" during this very dark chapter in Australia's history.
"He was sent home because he was a witness to a regime that involves torture and cruelty to innocent people," Senator Di Natale told reporters in Adelaide where the Greens were holding their annual conference.
"This is not what you expect from an Australian government."
Senator McKim defended his actions in trying to enter the Manus Island detention centre before being deported
Senator McKim said he went to Manus Island to mark the six-year anniversary of offshore detention and bring to the attention of Australian people and the rest of the world the terrible conditions refugees were living in.
He said he had been granted a six-month multiple-entry visa and had emailed PNG immigration officials asking for permission to visit the centre.
"They did not reply to my email, so I simply walked up on the public road to the gate of the centre and politely asked to go in," he told ABC television on arrival at Brisbane airport from Manus Island.
After being confronted by an immigration official he was then taken away by "heavily armed" police officers and was later issued with a deportation notice at his hotel and instructed to leave.
PNG Immigration says the senator did not make a formal request to visit the centre, but the Tasmanian senator believes an email to to the appropriate official, who he had previously dealt with, was sufficient.
"They said I demanded to be let in. I did no such thing," Senator McKim said.
"They said I didn't make a formal request. In fact, I did make a formal request. I've acted in actually the same way as I've acted on all of my other trips to Manus Island."
He also denied there was a presumption that he had the right to enter the facility on the basis he was a foreign elected official.
"There is a veil of secrecy that settles over what's happening on Manus Island, what's happening on Nauru," Senator McKim said.
"People like me, and journalists who want to expose the truth, are simply prevented from doing so. And that is what enables the terrible human rights abuses that we've seen over the last six years."
Australian Associated Press