Senator McKim will visit the asylum seekers who remain on the island, and to check conditions of facilities there.
The Department of Home Affairs confirmed to SBS News this week that 457 asylum seekers remain in Papua New Guinea.
"I'm going there to … remind the men on Manus, and the women and men on Nauru, that they've not been forgotten and that millions of Australians support them and don't support what's been done to them," Senator McKim said.
He said he expected to see conditions have deteriorated since his last visit in 2017, based on recent reports of a spike in suicide attempts.
"The situation is absolutely dire and it's been six long and bloody years for innocent people who stretched out a hand to Australia asking for our help," he said.
- Nick McKim, Greens senator
It's been six long and bloody years for innocent people who stretched out a hand to Australia asking for our help.
"This is a dark and bloody chapter in Australia's story. It's time this chapter is drawn to a close and the men on Manus, and men and women on Nauru, get the freedom and safety they so desperately need and they so deserve.
"And the six-year anniversary, I hope, provides all Australians with a chance to reflect on lives that we have destroyed and in some cases ended in the pursuit of base domestic political agendas in this country."
Six years on Manus
Kurdish refugee Behrouz Boochani said six years on Manus Island have fundamentally changed him.
"I was a hopeful and positive person but after living in circumstances like this, my body is damaged, my soul is damaged," he told SBS News last week.
"But it's not only me – hundreds of people are physically and mentally damaged."
The regional processing centre on Manus Island closed in 2017 but many asylum seekers including Mr Boochani now live in camps in the main town of Lorengau.
He said the post-election spike in suicide attempts was just one of the reasons why the situation had become out of control.
"Most people are so depressed they are locked up in their rooms ... They don't have hope."
And Mr Boochani said many asylum seekers on the island have become so desperate they don't even want to come to Australia anymore.
"What is important for us is to get freedom and it is not necessary that we go to Australia," he said.
"Of course there are a few people who want to go to Australia because their families are living there but most people only want to get off this island."
The options for asylum seekers
A spokesperson for the Department of Home Affairs updated SBS News on the situation in Manus.
"There are currently 457 [asylum seekers] in Papua New Guinea, 117 of which have been found not to be refugees," the spokesperson said.
"Refugees in Papua New Guinea are able to settle permanently in the community, as many have done, or apply to resettle in the US."
In 2016, then-prime minister Malcolm Turnbull announced a new resettlement deal with the US where it would take 1,250 refugees held on Manus Island and Nauru in exchange for Central American refugees coming to Australia.
But advocate have criticised the rollout and pace of the deal, with around 580 refugees settled to date.
As for those who are not recognised refugees, the Home Affairs spokesperson said they "may voluntarily return home or to another country in which the individual has a right to reside".
Although many asylum seekers claim these are not safe or realistic options.
"[And] no one subject to regional processing arrangements will be resettled in Australia," the spokesperson said.
- Home Affairs spokesperson
No one subject to regional processing arrangements will be resettled in Australia.
"Transferees continue to be provided with a range of health, welfare and support services, including extensive physical and mental healthcare provisions, free accommodation and utilities, allowances and employment services."
Asylum seeker advocates across Australia remain unconvinced that those in offshore facilities are receiving proper care and viable options to leave.
"Enough is enough," said Chirs Breen, co-organiser of Refugee Action Coalition.
Eighteen different regional and metropolitan groups will hold actions around the country from Thursday to Sunday.
"We will be calling on the Morrison government to bring the refugees on Manus and Nauru to safety in Australia, where they can get the care and security they requested in 2013."
Mr Breen said his organisation and others were "hugely concerned" about the post-election spike in self-harm incidents.
He said many of these individuals are "vulnerable people who are fleeing torture, war, all sorts of persecution. And they have been imprisoned for six years".
"There is no good reason to keep these people on Manus and Nauru ... The cruelty, the brutality has to end."
'A very hard-line decision'
After a major spike in boat arrivals, then-prime minister Kevin Rudd announced on 19 July, 2013 that all asylum seekers who came to Australia by boat would be sent to Manus Island.
"I understand this is a very hard-line decision. I understand the different groups in Australia and around the world will see this decision in different ways," he said at the time.
"But our responsibility as a government is to ensure we have a robust system of border security and orderly migration on the one hand, as well as fulfilling our legal and compassionate obligations under the Refugees' Convention on the other."
Weeks later, the Labor government announced a similar arrangement with Nauru.
After Mr Rudd's short second tenure, the Coalition continued the hard-line stance.
Under the Coalition's Operation Sovereign Borders, hundreds of asylum seekers have been transported to offshore detention centres rather than coming to Australia.
A brief history of offshore processing
The boat arrivals slowed and then-prime minister Tony Abbott credited Operation Sovereign Borders.
"In being magnificently successful, we have saved the lives of hundreds of people who might otherwise have been expected to drown at sea," he said in 2015.
But the United Nations and other human rights groups continue to criticise the camps' conditions and the long detention periods.
"Australia's offshore processing centres are unsustainable, inhumane and contrary to human rights," material from the UN High Commissioner for Human Rights says.
Readers seeking support and information about suicide can contact Lifeline 24 hours a day online and on 13 11 14. Other services include the Suicide Call Back Service on 1300 659 467, Beyond Blue and Kids Helpline (for people aged five to 25) on 1800 55 1800. More information about mental health is available at Beyond Blue.