Sydney: Papua New Guinea’s opposition has launched a legal challenge against the government’s decision to take asylum-seekers from Australia, saying refugees are held illegally in inhumane conditions.

Canberra has been sending boatpeople to PNG’s remote Manus Island, formerly used by Australia to process asylum-seekers under conservative ex-leader John Howard’s so-called “Pacific plan”, since late 2012.

In a statement received on Monday, PNG’s opposition leader Belden Namah said his lawyers had sent Prime Minister Peter O’Neill a summons to appear in court to defend the agreement, which he claims is unconstitutional.

“We challenge the right of the government to force people seeking refugee status in Australia to enter Papua New Guinea to be illegally and indefinitely detained under inhumane conditions,” Namah said.

“We challenge the right of the government to make this arrangement with the government of a foreign nation, again in contravention of our constitution.”

Namah said he would “take this matter as far as necessary to ensure that the values of our nation’s constitution are upheld”.

“We are filing injunctions to have the current detainees released, and to prevent the government from receiving or detaining any more asylum-seekers from Australia,” he said.

Australia last year decided to send asylum-seekers to Manus and the tiny Pacific state of Nauru in a bid to deter others from making the risky sea crossing from Indonesia, a route which has cost hundreds of lives in recent years.

Canberra insists all asylum-seekers sent offshore are treated humanely, but the settlement at Nauru has been criticised by rights group Amnesty as “appalling” and likely in breach of its obligations to refugees.

A spokesman for Australia’s Immigration Minister Chris Bowen said the latest issue was a matter for the PNG government.

“We are not going to comment or speculate on PNG politics,” the spokesman said.

The decision to send refugees offshore was made as the centre-left Labour government of Prime Minister Julia Gillard struggled to deal with an influx of asylum-seekers arriving by boat, with just over 17,200 arriving in 2012.

So far in 2013, 159 boatpeople have arrived on three separate vessels.