Foreign Minister Julie Bishop spoke to her Indonesian counterpart Retno Marsudi on Sunday evening while Prime Minister Tony Abbott has written to President Joko Widodo to again plead for the executions to be halted.

"Bali Nine" drug traffickers Myuran Sukumaran and Andrew Chan could face the firing squad as early as Tuesday, along with others from Brazil, Nigeria, the Philippines and an Indonesian prisoner.

Bishop said she had no official advice on when they might be killed, but "it could be April 29".

She said the men should not be executed while legal questions remain.

"I should point out that Mr Chan and Mr Sukumaran's lawyers are pursuing action before the Constitutional Court in Indonesia," said told ABC radio.

"And there's also a separate investigation underway by the Indonesian Judicial Commission into claims of corruption into the original trial and both of these processes raise questions about the integrity of the sentencing and the clemency process.

"I've asked foreign minister Marsudi that no action be taken in relation to the proposed executions until these legal processes have been determined," she added.

On Monday, Fairfax Media published allegations of corruption by the judges who sentenced the pair in 2006, claiming they asked for more than one billion rupiah -- around Aus$133,000 at the time -- to give them a prison term of less than 20 years.

It cited their then Indonesian lawyer, Muhammad Rifan, who claimed a deal fell through after intervention by Jakarta, which allegedly ordered they be handed the death penalty.

He said he decided to go public given the executions were imminent and the judicial commission, the Indonesian body that safeguards the probity of judges, had yet to complete its investigation into the alleged requests for bribes.

"This is an extraordinary situation because it is about lives. If they are dead they cannot be brought back again," he said.

At least one of the judges in the case denied to Fairfax there had been political interference or negotiations about bribes.

Todung Mulya Lubis, a lawyer for Chan and Sukumaran, told reporters the executions must not proceed until all legal avenues were exhausted.

"People should not be executed if the judgement came out of a defective process," he told reporters near the high-security prison island of Nusakambangan, where Indonesia puts condemned prisoners to death, referring to the claims surrounding the sentencing.

He added that if the allegations were proven at the judicial commission "the whole judgement must be annulled".

Bishop again warned Indonesia that its international standing could be damaged by the executions.

"I have made the point publicly and privately that this could harm Indonesia's international standing and when the secretary-general of the United Nations weighs into the debate I think that this is a global issue," she said.

UN chief Ban Ki-moon added his voice to appeals for the convicts to be spared on Saturday.

Sukumaran and Chan, along with the others facing the firing squad, recently lost appeals for mercy to Widodo, who has taken a hard line against drug traffickers and refused to back down despite mounting international criticism.

Asked what the consequences would be for bilateral relations if the executions are carried out, Bishop was non-committal.

"I don't want to speculate at this point about the consequences if these executions go ahead because my focus currently remains on doing everything I can to secure a stay of execution," she said.

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