I was smuggled into Australia's offshore detention camp on Manus Island in Papua New Guinea under the cover of darkness. Manus Islanders arranged the visit. They wanted us to bear witness to, and document the truth of the Australian Government's treatment of the men it has detained for more than four years.

It was the first time since the centre reopened in 2013 that video cameras have been inside without Government supervision. Despite years of experience working in humanitarian response in places like South Sudan, nothing prepared me for what I'd see.

It was so oppressively hot we could hardly film indoors because the humidity fogged the camera lens. Men drag their mattresses outside to escape the sauna-like conditions in cramped sleeping quarters that are shared by up to 40 men.

We saw evidence of months, if not years, of neglect of the facilities. Refugees showed us the handful of filthy and unusable showers and toilet facilities that hundreds of men have shared, and told us about the resulting outbreaks of ongoing skin infections and gastro.

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With hardly any food, many of the 600 men imprisoned in the centre looked listless. As our battery-powered studio light flooded the otherwise inky darkness, it attracted some attention from the men, but many remained motionless, almost unresponsive.

We returned to Australia and the smuggled footage made headlines around the globe. It lay bare what successive Australian governments have done to these men.

At the same time Jacinda Ardern reaffirmed New Zealand's offer to help resolve the crisis by resettling 150 men. Prime Minister Malcolm Turnbull refused, saying he wants to prioritise resettlement of up to 1250 people from Manus and Nauru to the United States, under a deal struck with the former Obama administration.

But resettlement in the US seems to be a distant glimmer of hope for a few, rather than the silver bullet Turnbull would have us believe it is.

As Turnbull and Ardern met on the sidelines of the East Asian Summit last week, it was a year to the day since the US deal was announced. A year on, only 25 from Manus Island have been taken to safety in the US.

As we wait, the men in the Manus Island detention camp are without food, water, electricity and medical attention. The Australian Government and its contractors walked away from the camp three weeks ago, flicking the switch on the way out and demanding the men move to "transit centres" elsewhere in the town of Lorengau.

They refuse to leave one place of detention for another. The men also cite a record of violence as a reason not to leave. And this is accurate. They face a hostile PNG Defence Force, a rightfully angry community, and memories of lethal attacks remain fresh.

When we toured the camp, the refugees showed me bullets holes from when the Defence Force opened fire on the centre on Good Friday last year, injuring nine people.

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I heard a heart-wrenching account of the night Reza Barati was killed during a riot in 2013, when guards and angry locals stormed the camp.

We also heard from locals, horrified at the way Australia has treated the refugees, and furious at the governments of PNG and Australia for playing refugees and locals, like pawns in a political game.

The local people never consented to the Australian Government dumping 600 men in their community. One local leader, concerned about further violence , likened it to an "invasion" by Australia.

New Zealand should be commended for its offer of resettlement and should continue to explore diplomatic channels with Australia, and PNG, to implement it.

But as Ardern herself concedes, resettlement takes time. The only way to avoid serious injury, illness or death, is immediate evacuation of refugees and people seeking asylum from Manus while the governments of safe countries around the world - NZ, the US, Canada - work to resettle the men successive Australian governments have detained illegally.

I invite the New Zealand people, and Prime Minister Jacinda Ardern, to stand with us in publicly calling on the Australian Parliament to work together to immediately evacuate the men detained on Manus. The responsibility is firmly on Australia, but there is an obligation on us all to act to preserve life. Evacuate now.

•Matthew Phillips is an Australian human rights campaigner with GetUp, an organisation promoting a "progressive" Australia and democratic participation. He manages a campaign against corporate involvement in mandatory detention of asylum seekers and has worked in South Sudan with Oxfam.