Papua New Guinea Prime Minister Peter O'Neill says projects agreed to by Australia in exchange for expanding its asylum seeker processing capability have not been costed.

Australia and PNG agreed on Friday that asylum seekers travelling to Australia by boat will be turned away, processed and potentially resettled in PNG.

In exchange, Australia has a agreed to a raft of infrastructure programs in the university, roads, health and law and order sector.

"The costings for the Ramu-Madang highway has not been done, design and costings for of course Lae hospital has not been done. So it costs into millions of Kina," Mr O'Neill told journalists in Port Moresby upon his return from Brisbane on Monday.

"But I cannot just simply give you a figure that is imaginary."

When asked where in PNG asylum seekers will be resettled under the plan he replied: "We'll get there when we start processing them."

Mr also O'Neill declined to nominate an upper limit to the number of asylum seekers he expects to be processed in PNG.

Manus MP Ronnie Knight has said he was told up to 3000.

"It'll be down to what the capacity on the ground can take," Mr O'Neill said.

"Listen, I cannot give you a figure, I cannot know what's going to happen in the future.

"We are hoping this is going to stop the non genuine refugees and asylum seekers coming into our region. I don't think the numbers are going to be as big as what we think its going to be."

He cited Australian media reports of people smugglers saying the expanded processing plan had already had an impact.

"You will see genuine people travelling, but non genuine economic migrants will fall off," he said.

PNG's opposition whip Tobias Kulang on Monday took to the national newspapers to blast the plan.

He said Mr O'Neill should have explained the scheme to the nation's parliament before announcing it in Australia.

"I am convinced we are far less equipped to handle the situation at this stage, especially to respond and to accommodate these asylum seekers.

"Australia must also come out and explain what and how it plans to assist PNG manage this affair."

Church leaders have also condemned the plan.