Detainees in the centre took cover after a large group of men stormed the gates of the centre about 6:30pm local time on Friday.
The detainees said Australian staff ran for cover after locking the gates.
Some sheltered inside the centre with asylum seekers while others were seen running for a boat moored nearby.
The shooting was reportedly in response to an earlier fight between asylum seekers and PNG Defence Force personnel from the adjacent Lombrum Naval Base.
Australia’s Department of Immigration and Border Protection said in a statement one man had been injured by a rock thrown into the centre but other residents were safe.
“There are reports PNG military personnel discharged a weapon into the air during the incident,” the statement said.
“No-one was injured.”
But detainees sent photos from within the centre showing bullets and bullet damage to their accommodation.
They said at least 100 rounds were fired during the incident.
Iranian refugee Amir Taghinia was on a bus being driven into the centre when he heard shots.
“It’s crazy, many people are in shock,” he said.
An Iranian asylum seeker, who wished to remain anonymous, said Australian security staff were sheltering inside the complex with the detainees.
“Security staff were inside, they were hiding with us,” he said.
He said the attackers used high-powered weapons and left bullet holes in building and destroyed a light fitting.
“They were repeatedly shooting and a couple of rooms have been shot,” he said.
Asylum seekers inside the centre said they did not know if anyone had been shot. (Courtesy ABC News)