SYDNEY (AFP) - Two Australian drug smugglers on death row in Indonesia will be transferred on Wednesday from their jail on Bali to an island off Java where they will be put to death, an official said.

Andrew Chan and Myuran Sukumaran, the ringleaders of the so-called "Bali Nine" drug trafficking gang, and officials escorting them will be flown in two military aircraft, said Momock Bambang Samiarso, head of the Bali prosecutor's office.

Officials are yet to announce a date for their executions, but the transfer indicates it is imminent. Indonesian authorities must give convicts 72 hours notice before they are put to death.

The men, in their early 30s, were convicted of trying to smuggle heroin out of Indonesia in 2005 and sentenced to death the following year. They recently lost their appeals for presidential clemency, typically a death row convict's last chance to avoid the firing squad, and are expected to be put to death soon.

Canberra has mounted a sustained diplomatic campaign to stop the executions from going ahead, but President Joko Widodo has been a vocal supporter of the death penalty and has refused to change course.

Following a coordination meeting with other officials on Tuesday, Samiarso told reporters: "The coordination has been completed. (The transfer) will be carried out tomorrow."

The Australians are being transferred to Nusakambangan island, which is home to several prisons. Five people, including foreigners, were put to death on the island in January.

The Australians are among a group of foreigners, including a Frenchman and a Brazilian, who have lost their appeals for clemency and are facing imminent execution.

Australian Prime Minister Tony Abbott has made repeated appeals for Chan and Sukumaran's lives to be spared. Chan's brother, Michael, called on Widodo to show mercy in an appearance on Indonesian television at the weekend.

"Andrew is a changed man from 10 years ago," he said, urging Widodo to give him a "second chance".