Greens senator Nick McKim is heading home having been prevented from visiting the Manus Island detention centre despite claims the facility is "open".
The Greens' immigration spokesman is leaving Manus Island hoping one day a royal commission will lead to apologies and amend for the treatment of detainees.
A disappointed Nick McKim was prevented from visiting the regional processing centre on the island during a trip this week.
The Papua New Guinean government says an investigation into the Good Friday shooting was still ongoing.
"This is supposed to be an open centre, that's what (Immigration Minister) Peter Dutton always says, it's what Malcolm Turnbull always says but it appears it's open until a commonwealth parliamentarian not from the government party wants to go and have a look," Senator McKim told AAP on Thursday before heading home.
He was able to speak to detainees outside the centre, including some who were eyewitnesses to the events leading up to Good Friday, as well as the province's police commander David Yapu and local MP Ronnie Knight.
"The story is consistent across all of those people that the shootings came about due to an escalation of tensions due to the use of a soccer field," Senator McKim said.
"The only person with an alternative version of events is Peter Dutton who's lying about what led to those shootings."
Mr Dutton has repeatedly stated PNG soldiers opened fire on the detention centre because they were concerned about the welfare of a five-year old boy who was allegedly led into the compound.
But Senator McKim said it's clear having spoken to police and detainees that Mr Dutton can't guarantee the safety of anyone in the camp.
"I just hope one day there is a royal commission into this so that reparations can be made, apologies can be made, people can be held to account and we can do everything we can to make sure this sort of thing never happens again," he said.
Senator McKim has learnt that about 300 detainees from the centre have undergone the first stage of vetting under a US deal to accept refugees and the second stage is starting this week.
But none have been given guarantees they will actually go, he said.
"Many of them want to hope that they could go to the US but their hopes have been destroyed by the Australian government so many times that they say to me 'we do not want to hope anymore'."
Senator McKim has a 12-month visa for PNG and hopes to return once the Good Friday investigation wraps up soon.
"Once that investigation is concluded the reason that was given to me by the Papua New Guinean government will no longer apply," he said.