The footage, filmed by the activists on Sunday night (November 5) shows blocked toilets and filthy bathroom facilities with no running water, and men sleeping outside on folding tables in sweltering conditions.
It shows the men have used wheely bins to catch and store water and also dug wells in search of ground water to use for drinking and cooking.
There are handwritten signs on the wall reading "Why do you exploit our weakness?" and "Shame on you".
Media access to the detention centre has been tightly controlled and the group claims it is the first time conditions inside the detention centre have been filmed without any government supervision.
The detention centre was closed nine days ago after a decision by the Papua New Guinea Supreme Court.
Approximately 600 men have refused to leave and move to other facilities, saying they fear for their safety if they do so.
The ABC was granted some access just before the camp closed last week.
On Thursday the PNG authorities gave the men 48 hours to leave voluntarily or they would face forcible removal or arrest. Authorities then began dismantling the high security fences around the camp.
Food, water and power to the site were cut off nine days ago.
In the footage, one detainee explains why they wanted the activists to film the conditions they are living in.
"We as the detainees and refugees, we did our best to send our voice to the world, but we know that ... people don't believe what we are doing. Only the minority are believing on it.
"This for us, it is a historical moment. Since 2001 when Australia made an agreement with the Papua New Guinea government to open this detention centre on Manus, they did not allow or give access to any media to come inside the detention centre.
"In the day it gets so hot that you can not even stay here for five minutes.
Another detainee says on camera, "There is no water here ... because of the dirtiness and the filthy place that these toilets are, many people have gastro problems and got very ill because of using these toilets. We had 20 people in a day many times having gastro issues."
'Death is inevitable in coming days'
Get Up!'s human rights co-directors Shen Narayanasamy and Matthew Phillips said they were smuggled in to the camp with videographer Paul Mackay at night by local Manus Islanders "who are increasingly opposed to the Australian government's treatment of the refugees".
"Death or serious illness is inevitable in coming days. These men are sick, thirsty and hungry. The conditions are appalling and it's obvious you wouldn't choose to stay here if you thought you could be safer elsewhere," Ms Narayanasamy said.
"There is a great threat of violence from the PNG army and the Manus Island community has made it very clear it doesn't want the men to move into accommodation closer to town.
"The men face a slow death if they stay in the detention camp, but outside they believe they will rapidly be in danger. It is a humanitarian emergency."
Refugees are 'squatting': minister
The activists blame the Australian government for the standoff, which they said is a humanitarian crisis.
"Only the Australian government has the capacity to evacuate the camp," Mr Phillips said.
But on Friday Cabinet minister Christopher Pyne said the refugees who remain in the centre are squatting.
"All those people in Manus Island who are at that detention centre are effectively squatting there," Mr Pyne said.
"They could go to East Lorengau, West Lorengau, Hillside House. They could go to Nauru, the United States, they could go home to their own countries — and many in fact have done so."
Earlier this week the PNG Supreme Court rejected an application by a refugee to have services to the camp restored, and now a lawyer representing one of the refugees has lodged an appeal against that decision.
