POOR parenting is the cause of the rising number of young people in the juvenile justice system, says Victorian Police Commissioner Simon Overland.

''Parenting has become something of a lost skill,'' he said at the launch of a fund-raising event for the Whitelion foundation, which finds mentors and jobs for young offenders. ''We've lost the ability to say no.''

The number of young people in detention and under community-based supervision for their crimes has increased across Australia in the past four years, according to figures to be released today by the Australian Institute of Health and Welfare.

Between 2005-06 and 2008-09, children aged 10-17 under supervision orders and detention increased from 2.2 per 1000 to 2.5. Half of the young people in detention in Australia are Aborigines and Torres Strait islanders, even though only 5 per cent of young Australians are indigenous. Victoria's under-age detention rates (0.1 per 1000) and community-supervision rates (1.5 per 1000) were the lowest in Australia.

While only one in 10 offenders in detention are female, more young women are committing crimes. Mr Overland said one cause was young women forced on to the street by sexual abuse.

''Sexual assault, probably incest,'' he said. ''It's a lack of safety in their home, either through sexual assault, experiencing family violence or emotional abuse. That would be a key driver behind [a rise in crime].''