JAKARTA (AFP) -- The mothers of two Australian drug smugglers on death row in Indonesia tearfully begged authorities on Monday to "spare our sons' lives", as their lawyer revealed plans for a last-gasp legal bid.

Andrew Chan and Myuran Sukumaran, ringleaders of the so-called "Bali Nine" drug smuggling gang, were arrested in 2005 and sentenced to death the following year after being caught trying to traffick heroin out of Indonesia.

They lost a legal bid last week to have their cases reviewed, effectively dashing their final hope of avoiding the firing squad. Jakarta has said they will be executed this month.

Indonesia last month executed six drug offenders, including five foreigners. On Monday the families of Chan and Sukumaran, who in recent days have been on the resort island of Bali where the men are held, visited Jakarta to appeal for authorities to spare the lives of the men, who are in their early 30s.

"We beg you for mercy for our sons, we beg you to spare our sons' lives, we beg you that they not be killed," said Sukumaran's mother Raji Sukumaran, sobbing as she read from a statement signed by her and Chan's mother, Helen Chan.

At an emotionally charged press conference, the women called on President Joko Widodo, the vice president, the attorney general and "the Indonesian people to show mercy on our children".

"I understand the serious crime my son committed. They are both very sorry for this. We are very sorry for this. They are not the same people who committed that crime almost 10 years ago," Raji Sukumaran added.

She said the pair had been rehabilitated and were now the "driving force" behind improving the notorious Kerobokan jail on Bali, through starting programmes that ranged from painting and photography to dance and sewing.

Chan's brother Michael, who attended the event, added: "If we can show the president that these guys have done such an amazing job with what they've had to deal with, I just want them to have that second chance to show that to other people."

Earlier, their lawyer Todung Mulya Lubis said he planned one last attempt to save their lives by challenging in an administrative court Widodo's decision not to pardon them.

Both men lost their appeals for presidential clemency in recent months. The legal move has been rarely attempted before, but Mulya said he did not believe Widodo could simply reject the men's pleas on the basis of a drug emergency.

Widodo has been a vocal supporter of capital punishment and warned Indonesia was in a state of emergency due to drugs, with dozens of people dying every day.

"I don't think the president can use that blanket argument to refuse all the clemency applications," he said, adding: "It is unfair, it is unjust, it is unacceptable."

The lawyer added the challenge was "probably the only legal recourse left for us" and he hoped the attorney general would exclude the pair from any executions this month while the legal process was ongoing.