Police in Papua New Guinea are appealing for help to capture one of the escaped killers of Iranian asylum seeker Reza Barati.

Manus Province police commander David Yapu said Joshua Kaluvia escaped for a second time from the prison on the island on February 18, during a water shortage at the jail.

Key points: Iranian asylum seeker Reza Barati was killed inside the Manus Island detention centre in February 2014

Iranian asylum seeker Reza Barati was killed inside the Manus Island detention centre in February 2014 Joshua Kaluvia and Louie Efi were found guilty of his murder in 2016

Joshua Kaluvia and Louie Efi were found guilty of his murder in 2016 Kaluvia escaped jail for the first time in 2016 ahead of his sentencing

"When there was a problem with the water supply the prison warders had to escort them down to the shore to have their shower at the beach," Senior Inspector Yapu said.

"Whilst out at the beach he managed to escape."

Kaluvia is one of two men convicted of murdering Barati inside Australia's detention centre on Manus Island in February 2014.

The former Salvation Army worker is serving a five-year sentence, along with former G4S guard Louie Efi.

His sentencing was delayed when he escaped in March 2016 and spent two weeks on the run.

Police say latest escape is concerning

"He's a dangerous and high-risk criminal element," Senior Inspector Yapu said.

"My fear is that if he's on the run he can cause threats to the community. In other words, he's dangerous to the community.

"I've appealed to the community and the people of Manus to assist us to provide information of his whereabouts so we can soon arrest him and put him back behind the bars. Otherwise, if he's still on the run, he's a threat. He's a threat to the community."

Two of the witnesses to the murder of Barati remain in the detention centre on Manus Island.

Witness fears for safety

The main witness — Barati's roommate Benham Satah — said he was terrified.

"I am going crazy, I don't know what to do," he said.

"Police are searching for him, but I can't sleep or eat unless he is detained again. I have to be on alert always."

Mr Satah received a number of death threats after giving evidence about the killing and was promised protection by the judge in the murder trial.

Police said they had advised the centre's management and the contingent of officers stationed there to watch for Kaluvia and protect the witnesses.