Sir Richard Branson has added his voice to the calls for clemency for two Australians on death row, as an Indonesian minister warns of a renewed influx of asylum seekers if relations between the two countries are damaged over the Bali Nine duo.

The founder of Virgin spoke out this morning in his role as a member of the Global Commission on Drug Policy.

Sir Richard and fellow commissioners have written to Indonesian president Joko Widodo, saying the impending execution of 10 prisoners for drug crimes — including Australians Andrew Chan and Myuran Sukumaran — is a barbaric and inhumane form of punishment.

He said the commission's recommendation for clemency came from evidence based on different countries' approaches to drugs.

"What we have learned is that treating drugs as a health issue – not as a criminal issue – it actually helps lower the number of drug deaths," he told ABC News Breakfast.

"It limits the spread of infectious diseases like HIV and AIDS or hepatitis C, it reduces drug-related crime, and it allows people who struggle with addiction to become useful members of society again.

"We would love to be able to show the Indonesian government how countries like Portugal and others have completely overcome their drug problem by taking a very, very different approach."

Sir Richard said he was prepared to travel to Indonesia to put his case to the government.

"I am willing to go and get on a plane today, tomorrow, as are the other commissioners," he said.

"What we want to do is try to be helpful. All of our research is evidence-based and we have studied different national approaches in great depth."

Australia reminded of asylum seekers in Indonesia

But within Indonesia, defensiveness over the issue is growing.

Almost every statement made by Prime Minister Tony Abbott and the Australian Government has been analysed and reported widely in Indonesia in recent weeks, some of it inaccurately.

The perception that foreign aid was being threatened has caused offence, and so too has the incorrect impression that Australia might have threatened travel boycotts to Bali if the Chan and Sukumaran executions went ahead.

One colourful nationalist minister, Tedjo Edhy Purditjatno, has leapt on the controversy and pondered out loud the possibility of retribution.

The coordinating minister for political, legal and security affairs reminded Australia there were 10,000 would-be asylum seekers in Indonesia.

"Imagine if we let them all go to Australia," he said.

He did not directly threaten to allow that to happen, but the issue of maritime border security is on display in Cilacap, near Nusakambangan island, where the death row inmates are being held.

Unusually, two large navy warships are docked there and have patrolled waters around Nusakambangan prison and along the southern coast of Java.

Families make second visit to Nusakambangan island

None of this is of any concern to the Chan and Sukumaran families.

An eight-member delegation from both families, together with consular officials, today made their second visit to Nusakambangan, ahead of tomorrow's crucial legal challenge to their death penalty.

Sukumaran's brother, Chintu, again called on Mr Widodo to show mercy.

"All we ask is that they be allowed to spend the rest of their lives in prison and not be executed," he said.

"Our family remains hopeful that the president will get to see how much Myuran and Andrew have done inside the prison to help the Indonesian people."

Meanwhile, a Frenchman on death row has been transferred under heavy police guard from Nusakambangan island, the day before he starts a last-ditch court bid to avoid the firing squad.

Serge Atlaoui has been detained on the island since he was sentenced to death in 2007 on drugs charges and had an appeal for clemency rejected in January by Mr Widodo.

The father of four was arrested near Jakarta in 2005 in a secret laboratory producing ecstasy.

Imprisoned in Indonesia for 10 years, he has always denied the charges, saying he was installing industrial machinery in what he thought was an acrylics factory.