A man believed to be one of Australia's worst sex offenders has admitted to molesting children across south east Asia and keeping thousands of photos and videos of the abuse.

For more than 16 years Melbourne man Boris Kunsevitsky, 52, abused up to 47 young boys across four countries in south-east Asia - many of them living in impoverished slums.

Kunsevitsky was living in Singapore in 2016 but was being monitored by Australian police.

When he returned home in 2017 officers seized his phone and found explicit child exploitation material, The Age reported.

Over 16 years Melbourne man Boris Kunsevitsky (pictured), 52, abused up to 47 young boys across four countries in south-east Asia

Kunsevitsky wasn't allowed to return to Singapore and police had his belongings shipped to Australia.

When the horde arrived police found a laptop and two hard drives, containing thousands of horrific images of child sexual abuse.

More than 3,300 pictures and videos were found dating back to 2001.

From the evidence police were able to deduce that Kunsevitsky had abused or sexually assaulted up to 47 underage boys.

Most were believed to be teenagers, but some victims were 12 years and under and many were living in impoverished slums.

Among the victims was a Melbourne teenager who was abused continuously between 2004 and 2005.

While living in Australia, Kusenvitsky groomed the 14-year-old boy online while pretending he was 19.

He repeatedly abused the boy, and once he even abused the boy at him home when his parents were out.

Kunsevitsky filmed the encounters and shared them online, before they were investigated by German police who alerted Australian authorities.

The victim, now a man in his 20s, told the Supreme Court in Melbourne that Kunsevitsky made him believe he was was the only person who cared for him.

Kunsevitsky was living in Singapore at the time, however when he returned to Australia to visit his family in 2017, police had been investigating him for a year

He said he was left confused and distanced from everyone around him, so he turned to drugs and even tried to commit suicide when he was just a teenager.

'I lost almost everything and everyone in my life and this has been truly catastrophic,' he told the court.

Many of the other young victims were from the Philippines, Singapore and one lived in Indonesia.

Kunsevitsky had sex with at least 25 of the underage boys, while others performed sex acts on him and others.

The 52-year-old has been classed one of the most serious child exploitation offenders in Australia and is facing decades behind bars.

In addition to the 3,300 images and videos he collected himself, he also downloaded more than 37,000 child porn images.

Kunsevitsky had sex with at least 25 of the underage boys, while others performed sex acts on him and others

The court heard how Kunsevitsky would travel throughout south-east Asia for 'work' but was actually engaging in sex tourism.

Prosecutor Krista Breckweg said the material often showed him engaging in sexual acts or instructing the boys to perform sexual acts on each other.

Kunsevitsky pleaded guilty to multiple charges, including maintaining a sexual relationship with a child, 25 counts of engaging in sexual intercourse with a child outside Australia, 12 counts of inducing a child to engage in sex with a child and child pornography offences.

Ms Breckweg said Kunsevitsky showed lack of remorse and the offences against the Melbourne victim were 'depraved and despicable'.

Kunsevitsky has been remanded in custody and is due to be sentenced at a later date.