Prime Minister Tony Abbott has declared "the way is closed" for people smugglers as the Government marks 100 days since a boat arrived in Australia's territory.

Standing next to a sign comparing the number of boat arrivals under Labor and the Coalition, Mr Abbott congratulated Immigration Minister Scott Morrison on progress with Operation Sovereign Borders, which has been tasked with stopping asylum seeker arrivals.

The Prime Minister said Mr Morrison had done an "outstanding" job.

"This is the result of ... full and methodical implementation of the policies that the Coalition took to the last election," Mr Abbott said.

"It is too early to declare that the job is done, but nevertheless I think we can safely say that the way is closed."

The Opposition said it was too early to "proclaim victory", but welcomed the reduction in boat arrivals.

The Government said with the monsoon season coming to an end, the number of boats attempting the journey to Australia might increase.

But Mr Morrison said the first two phases of the operation - 100 days of a reduction in arrivals and the same amount without any arrivals - had been successfully completed.

He said Operation Sovereign Borders would now enter a new phase as the Government focused on the 30,000 people still in immigration detention.

"We now go into the third phase where we move into the post-monsoon period and the risks are just as great," Mr Morrison said.

"We will maintain the intensity of all of our operations in all areas of Operation Sovereign Borders, both with our offshore processing, with what we are doing at sea and through our disruption and partnership operations all the way up through the region back to source.

"We need to work through the settlement arrangements there and the return of those who are found not to be refugees."

Mr Morrison has released figures showing 606 people have now either voluntarily returned or been forcibly returned to their place of origin since Operation Sovereign Borders began.

The Government says for the first time since 2008 the number of people returning home is exceeding the number arriving.

Opposition immigration spokesman Richard Marles said the arrangements struck by the previous Labor government had been the main factor in the slowdown in boats.

"It's too early to proclaim victory. This is not a footy match - this is not about scoreboards and banners and slogans. This is about serious public policy addressing a very, very complicated issue," he said.

Morrison refuses to comment on Manus Island investigation

Meanwhile, Mr Morrison has refused to comment on reports Papua New Guinea police are investigating two Australian suspects in the recent killing of an asylum seeker on Manus Island.

Fairfax Media has reported witnesses to the violent death of detainee Reza Barati in February have identified two employees from security contractor G4S.

Mr Morrison said it was not appropriate to discuss the police inquiry.

"That matter is still before the Papua New Guinean police and as I said I'll be in PNG this week to get a further update on where those investigations are at," he said.

"Our own inquiry is also continuing under Mr [Robert] Cornall and when those matters are brought to a conclusion, if there are any requests that are made obviously we'll work through those requests as appropriate."