A paedophile who was sentenced to 35 years in prison had forced his victims to smile as he raped them and filmed the abuse at his wife's parents' home, a court heard.

Boris Kunsevitsky, 53, was dubbed Australia's 'worst paedophile' on Wednesday after details of his sickening offences were laid bare in court.

The former paediatric nurse, from Melbourne, pleaded guilty to 59 offences including sexual intercourse with a child and producing child pornography outside Australia.

He admitted to documenting the sexual abuse of 47 boys across four countries in more than 35,000 photos and 4,800 videos.

Boris Kunsevitsky, 53, was dubbed Australia's 'worst paedophile' on Wednesday after details of his sickening offences were laid bare in court

Boris Kunsevitsky (pictured), 53, is among the worst paedophiles in Australian history

The footage was discovered in a hard drive in a folder labelled 'jailbait', which Supreme Court Judge John Champion called 'sordid' and 'difficult to look at.'

The court heard Kunsevitsky had filmed the boys as he raped them and even demanded they smile as they winced in pain.

'In one video you can be seen looking at the camera and smiling,' Judge Champion said, calling it 'particularly horrifying.'

Kunsevitsky, who carried out his abuse over a 15-year period between 2002 and 2017, even sexually assaulted one of his victims at the home of his wife's parents.

The rapist drove a 12-year-old boy, who he had met in an online chat room, to the house where he set up a camera and filmed himself performing sexual acts.

Judge Champion branded the abuse 'breathtaking in its scale,' adding that it represented 'the worst level of offending' he had ever seen.

The depraved acts took place in Singapore, Indonesia, Australia, with most of the abuse committed against boys in the Philippines.

There, Kunsevitsky made the boys perform sexual acts on themselves and each other, and filmed and photographed it.

'You chose destinations where you could easily and readily access young boys,' Justice Champion said.

'You are a child abuser of appalling... proportions,' the judge added.

Boris Kunsevitsky (pictured) produced more than 35,000 photos and 4,800 videos of abuse

Kunsevitsky also abused one boy in Melbourne between 2004 and 2005, with German police years later finding 55 images of the abuse in a folder labelled 'jailbait'.

Kunsevitsky's Australian victim went on to develop depression, anxiety and attempt suicide. He ended up $100,000 in debt because of a subsequent drug habit, lost his job and suffered a marriage breakdown.

'There is no sentence that I can pass that will undo the tremendous pain you have caused (the victim),' Justice Champion said.

Some of Kunsevitsky's overseas victims have never been identified.

Kunsevitsky, who was born in the former USSR, moved to Australia when he was 12 and went on to do graduate nursing training at the Royal Children's Hospital.

He then moved to Singapore in 1999 to work at a medical equipment firm and proceeded to travel around Asia sexual abusing boys, the court heard.

Kunsevitsky among the worst paedophiles in Australian history.

He abused more boys than Bernard McGrath who was jailed in 2018 after being charged with abusing 43 boys at a school north of Sydney in the 1970s and 1980s.