Bali police are investigating whether four foreign inmates, including an Australian man, drowned as they tried to dig their way out of Kerobokan Prison.

The inmates, including Perth man Shaun Davidson, fled from the notorious jail through a tunnel yesterday morning.

A police diver attempted to check the tunnel today but discovered it was unstable and flooded with water.

Badung Police Chief Yudith Satriya Hananta said police did find a fork inside but there was no sign of the prisoners.

There are now fears the inmates are trapped or possibly could have drowned.

Police are working to drain the tunnel as part of investigations.

Australian man Shaun Davidson has escaped from Kerobokan Prison. (Supplied)

Authorities have questioned prison guards and inmates and continue to monitor the island’s airport and sea ports in search of the escapees.

Davidson was sentenced to one year in jail in September for using a fraudulent travel document.

The Subiaco man had been in custody since April 5 last year and had just two months and 15 days remaining on his sentence.

Prison guards found a 50cm by 75cm hole behind a prison medical clinic, through which the inmates are alleged to have escaped.

The hole, police said, was 15 metres long and travels underneath the prison wall and emerges at a road running along Kerobokan.

All four men were serving time for various offences at Kerobokan's Bedugul block.

Davidson was jailed for using a fraudulent travel document. (AAP)

Balinese officials have released photos of the escapees. (Supplied)

Bulgarian man Dimitar Nikolov Iliev, 43, was serving a seven-year term for money laundering, while alleged fellow escapee Saye Mohammed Said - a 31-year-old from India - was in the midst of a 14-year sentence for drug offences.

Malaysian man, Tee Kok King, 50, is understood to be serving seven and a half years, also for drug offences.

Davidson also has a warrant out for his arrest on drug offences in Perth.

When the 33-year-old arrived in Bali in January 2015 he said he spent several months just "partying and boxing" before arousing the suspicion of authorities when he signed into a guest house in Kuta under a passport with the name Michael John Bayman - claiming the photo was taken when he was "chubby".

After his sentencing at Denpasar Court last year, Davidson said he spent his first few weeks in custody crammed into a small cell with 20 other people.

"It was built for 300 people, there is 1200 there ... It's pretty hard for some of the locals in there, if they don't have any money, you don't eat. They don't give you any food, they don't give you a bed. They don't give you anything."

Although his sentence was only for one year, he opted to spend an an extra five months in prison rather than pay the 10 million rupiah fine ($A10,000).

Despite overcrowding in Kerobokan, he said it wasn't the "living hell" he thought it would be.

"I guess I'm just lucky enough to have support from the outside," he said.

Prison escapes in Indonesia are not unusual.

Just last week, 76 inmates escaped Jambi jail in Sumatra after flooding caused the prison's walls to collapse.

In May, in another Sumatran facility, Pekanbaru, 442 prisoners made a run for it following allegations of guards charging people for cells in the chronically overcrowded facility.

In January 2013 there were reports of a prisoner escaping through Kerobokan's sewerage system in the early hours. He was captured that night at his wife's house.