They were vicious attacks that spread fear through Sydney's northern suburbs; two young girls approached in separate incidents who only just managed to escape the grasp of a stranger with a sinister motive.

But now police allege Sydney father Mark Anthony Hanson, 50, is also responsible for more sexual attacks on young girls in Sydney's north and north-west over a number of years.

The travelling paint salesman from Castle Hill has already been convicted of trying to abduct a six-year-old girl at South Turramurra in February 2012 and grabbing the throat of a 14-year-old girl and threatening to rape her at Westleigh two months later.

Hanson pleaded guilty to those offences in March and during sentencing six months later the court was told he had no previous criminal record.

But Hanson will appear before Parramatta Local Court on Thursday, charged with 18 other offences, including sexual assault and indecent assault. It is understood they related to six victims and four alleged incidents.

Some of those attacked are aged under 10 and police have not ruled out the possibility of other victims coming forward.

The new charges follow investigations by the NSW Sex Crimes Squad into reported similar incidents in Sydney's north and north-west between 2007 and 2011.

Hanson is serving seven-and-a-half  years jail, with a non-parole period of five years and three months, for the two attacks in early 2012. He was convicted of six charges, including child abduction and child pornography offences, in March this year.

In the first of those attacks, a woman, who was waiting to meet friends at a netball court, noticed a man driving slowly behind the six-year-old girl as she scooted down Canoon Road, South Turramurra. The witness then saw Hanson approach the girl behind a toilet block where she let out a "short, young, piercing scared scream". The woman ran over and Hanson ran off into the nearby bushland.

Two months later, Hanson grabbed a 14-year-old girl by the throat on a footbridge at Westleigh, threatened her with a Stanley knife and told her he would rape her.

The girl managed to break free and run home and raise the alarm.

While Hanson was previously unknown to police, security footage of his car near the Westleigh attack and mobile phone records put him at or near the scene of both attacks and brought him to police attention.

DNA taken from the T-shirts of both girls then matched Hanson's profile.

Hanson pleaded guilty to both attacks in the Parramatta District Court in March and was sentenced in September this year.