“They did not reply to my email, so I simply walked up on the public road to the gate of the centre and politely asked to go in,” he told ABC TV.
An immigration official allegedly took his passport and told him to leave, but he stayed put demanding to have his passport back.
“And he eventually brought my passport back, and then five minutes later I got bailed up by four heavily armed police officers.
“And they told me they'd be around to my hotel later that day to issue me with the deportation notice, and they did come around later and I was instructed to leave.”
The federal Greens politician repeated earlier comments that the incident reinforced the “veil of secrecy” surrounding offshore detention.
“I've done nothing wrong here,” he said.
“This is one of the reasons that offshore detention was established in the first place. So that it is impossible to hold officials in Papua New Guinea, and by extension the Australian government, including Minister Peter Dutton and Prime Minister Scott Morrison, it's impossible to hold them to account for what is happening, because it is impossible to find out exactly what is happening.”
Senator McKim’s visit to PNG was an attempt to check on conditions on the sixth anniversary of the Rudd government’s decision to prevent asylum seekers who arrived by boat resettlement in Australia.
Protests marking the occasion are scheduled for Saturday in major capital cities, where thousands are expected to attend, calling for an end to offshore detention.
Over 457 asylum seekers remain in Papua New Guinea.
In a statement issued on Friday, Papua New Guinea's chief migration officer Solomon Kantha said Senator McKim failed to seek “appropriate authorization” to visit facilities there, as he had done in the past.
Mr Kantha said the senator "did not go through appropriate channels" and therefore displayed "flagrant disregard for PNG".