At least five boys from Singapore were sexually abused by one of Australia's worst paedophiles.

Boris Kunsevitsky, 53, abused more than 40 boys during the 16 years he worked in Singapore, often travelling to nearby countries for sex tourism.

The Russian-born Australian had sex with at least 25 of them, taking more than 3,300 photographs and videos of the acts and distributing them online.

His other victims were made to perform sexual acts with each other, with videos showing Kunsevitsky telling some to smile and relax even as they appeared to be in pain.

Kunsevitsky, who was diagnosed with a paedophiliac disorder, pleaded guilty to 59 offences in the Supreme Court of Victoria in Melbourne on Tuesday.

They included sex with children outside Australia and the production and importation of child exploitation material.

Court documents revealed that more than 40 victims were identified from the thousands of recordings made by him. Many more victims could not be identified.

The documents, which were released to The New Paper yesterday by the Office of the Commonwealth Director of Public Prosecutions in Australia, revealed his victims were aged 10 to 17 at the time of the offences.

They also showed that he engaged in the "persistent sexual abuse" of five children outside Australia for three years from June 2010.

One boy was sexually abused on at least 10 separate occasions, and the others on at least three occasions each.

It was not clear if they were the victims in Singapore.

Kunsevitsky, who arrived in Singapore at the end of 2000, worked in sales, allowing him to travel regularly through South-east Asia to commit his crimes.

His victims were mainly boys living in slums in the Philippines. Others included a boy in Indonesia and one in Australia.

He was arrested when he returned to Australia in September 2017 to visit his family.

He was then working as technical director of a Singapore aesthetics company, Esthemedica, which he had joined in 2014.

Esthemedica terminated his employment after learning of his offences on Tuesday.

Its marketing manager, Mr Hoong Cheong Hon, said the firm was shocked to hear of the crimes, as Kunsevitsky said he he had been detained for tax offences and had to remain in Melbourne to recovering from an illness.

"We want to emphasise that the company does not condone such appalling acts against children or any individual for that matter," Mr Hoong told TNP.

"Our sympathy goes out to all victims who fell prey to his criminal acts and their families."

Kunsevitsky, who will be sentenced at a later date, is looking at decades in prison. Twelve of his 59 offences carry a maximum jail term of 25 years while the others range from 10 to 20 years.

Criminal lawyer Ravinderpal Singh of Kalco Law said Kunsevitsky was unlikely to be extradited to Singapore.

"Singapore authorities are unlikely to prosecute him for the same crimes as it would be double punishment," he told TNP.

But Mr Singh expects to see more cases involving extra territorial jurisdiction, where offenders are prosecuted for crimes committed outside their home countries, in the future.

A recent example is Operation Blackwrist, an international operation led by Interpol, in which nine offenders who sexually abused children were arrested and prosecuted in several countries, including Australia and Thailand.

Mr Singh said Singapore has been exercising extra territorial jurisdiction, most prominently in the area of drug consumption.

"We see many cases of Singaporeans who take drugs overseas but are prosecuted when they return home," he said.

"But I would say sex offences have been on the rise, because more such cases have been reported recently, when they were unheard of 15 years ago."