Seven years ago Irish priest Charlie Burrows witnessed the execution of two Nigerian heroin smugglers on Nusakambangan, the remote penal island where Andrew Chan and Myuran Sukumaran will also be shot dead.

It took the Nigerians up to eight minutes to die. "When they were moaning in pain I tried to sing Amazing Grace," Father Burrows said.

Their haunting deaths crystallised his opposition to capital punishment.

Later that year Father Burrows gave evidence in a legal challenge to the execution of the Bali bombers in the Constitutional Court.

"The legal team for Amrozi asked me to be a factual witness that death by firing squad is torture. There were eight judges in the Constitutional Court. Seven disagreed it was torture and one agreed."

Now Father Burrows is trying to get support for a challenge to the death penalty in the Constitutional Court on the grounds that waiting to be executed is also torture.

About 150 prisoners are on death row in Indonesia - Chan and Sukumaran have lived with that  Sword of Damocles for almost 10 years.

Father Burrows believes that living with the omnipresent threat of execution makes prisoners go insane.

He recently asked a psychiatrist to assess Brazilian cocaine trafficker Rodrigo Gularte, one of the nine other drug felons slated to be executed with Chan and Sukumaran.

Gularte was diagnosed with schizophrenia, bipolar, psychosis and paranoia. The Attorney-General has asked for a second opinion as the mentally ill and pregnant women cannot legally be executed.

Father Burrows says Brazilian drug felon Marco Archer Cardoso Moreira, who was executed on January 18, was also mentally ill.

"The fact that these people are going insane is proof that they are being tortured waiting," Father Burrows says.

He will meet MISEREOR, the German Catholic Bishops' Organisation for Development Co-operation, on February 26, in the hope they might support a legal challenge.

"We have to have a well-known, smart lawyer who might take our case on." Father Burrows says.

"We need a commitment for funding - embassies can be a bit slow in funding something like this because it might be seen as anti-government so we will have to go to a NGO."

Father Burrows, or Romo Carolus as he is known by locals in the Cilacap parish in Central Java, has been working in Indonesia for more than 40 years after a three-year stint in Bankstown, Sydney.

His parish includes the remote island of Nusakambangan, which has seven prisons. Many of the drug felons in these prisons are Catholics from countries including Brazil, Italy, Nigeria, Nepal and Cameroon.

In 2008 Father Burrows was asked to be the religious support person during the executions of Samuel Iwachekwu Okoye and Hansen Antonius Nwaolisa, who were convicted of smuggling heroin in 2001.

He walked with the men to the top of a mountain on the island, where a religious ceremony was performed. "I read tonight you will be with me in heaven. Antonius asked me to give his wife a handkerchief and a 100,000 Rupiah ($AUD10) note. He said he was thirsty and the guard gave him some water. He finished that off and then said: 'I want another'. The guard said 'You'll have a pain in the belly' and he laughed and said:  'I'll be dead in 10 minutes'. They put a hood over his face and the doctor put a black patch over his heart. Just before he was shot he said: 'Father are you still there? Could you take my shoes off and give them to my wife?'

The idea is the bullets hit the heart and the heart explodes. They should die quickly but he didn't."

Father Burrows spent time playing with Antonius' four-year-old son, Jodi, before his father was executed. He said the child knew something was amiss and kept knocking over toy soldiers and saying: "mati mati" (dead). "I said: 'But when you die you go to heaven' and I put the soldiers up again."

Father Burrows said Jodi will have grown up knowing the state killed his father. "It causes a vicious circle, it's going to cause a boomerang," he said.

Asked if he was scarred by the experience, Father Burrows said: "Because of that I was able to give evidence in the Constitutional Court".

He will not give up the fight to save those on death row, organising for the psychiatrist to assess Gularte and providing testimony for another Nigerian facing execution on Nusakambangan island.

He said like Chan and Sukumaran, the Nigerian drug felon, Okwudili Ayotaeze, had turned his life around. "He's very well thought of in the jail," Father Burrows said. "We are still working, still trying. You have to keep rattling the bloody gate."