The Greens senator Nick McKim says he was deported from Manus Island because he “simply stood on a public street and asked politely” to see conditions in a refugee and asylum seeker transit camp.

The Tasmanian senator arrived back in Australia on Saturday after he was told to leave Papua New Guinea on Thursday night.

McKim – who has a 12-month, multiple-entry visa – was visiting to mark the sixth anniversary of Australia’s resumption of mandatory offshore detention, when then-prime minister Kevin Rudd announced that all asylum seekers who arrived by boat would not be resettled in Australia.

McKim said he politely asked staff if he could enter the East Lorengau transit centre on Thursday, but was denied. Later that evening, PNG police visited his hotel room to tell him he was being deported from the country.

“I just walked up the public road to the East Lorengau transit centre and just politely asked to go in,” he said. “The immigration guy at the gates asked for my passport, I gave it to him. He said ‘Ok you can go now, but I’m keeping your passport’. So I said I wasn’t going anywhere until I got my passport back.

“He did give me my passport back. We left. Five or 10 minutes later we were just walking down the main road in Lorengau and got bailed up by four heavily armed police officers and instructed to get in the vehicle.

“They said: ‘We are deporting you’. I said I think you will need to make it more official than that. Later that night they came and instructed me to leave Manus and PNG.”

McKim told reporters in Hobart on his return on Saturday that things were “worse than it has ever been” on the island.

“And that’s because they’ve lost hope for the future,” he said.

He said the prime minister, Scott Morrison, could “simply agree with Jacinda Ardern” – who visited Australia on Thursday – that the refugees should be resettled in New Zealand.

“Ms Ardern has made it abundantly clear that her offer remains … He could bring an end to six years of human misery.”

On Friday, PNG’s chief migration officer, Solomon Kantha, said McKim “failed to request access to the centre through the appropriate channels” and lacked “appropriate authorisation”.

McKim said this statement was “riddled with inaccuracies”.

“They said I demanded to enter, I made no such demands. I simply stood on a public street and asked politely.

“They said I hadn’t gone through proper channels. Well I actually emailed their chief migration officer a few days ago, and formally asked permission to enter and never heard back.

“But the issue is not me, the issue is these guys have been imprisoned illegally for so many years and treated so badly. And we have a prime minister who with a stroke of a pen could end this torture.”