MILDURA speedway volunteer and ex-footy player has been sentenced to five years' three months' jail for the sexual assaults of a school girl in her home and a woman at her workplace.

Cameron John O'Brien was given a non-parole period of three years', 11 months, before the County Court of Victoria on Monday.

At a hearing three weeks ago, O'Brien pleaded guilty to sexual penetration of a child under 16, sexual assault related to a woman, seven charges of failing to comply with reporting conditions of the sex offenders register and two counts of possessing methamphetamine.

Judge Frances Hogan said the 29-year-old's offending was "predatory" and "manipulative", labelling it revolting behaviour given O'Brien knew the 15-year-old girl was extremely vulnerable at the time.

The sexual assault was committed last September in the teenager's own bedroom after O'Brien went to her house to see the girl's auntie.

At the time he was on bail for the sexual assault of a woman in July 2019 and was a registered sex offender on a community corrections order which he was sentenced to for the sexual assault of different teenager in 2018.

During the 2019 assault, O'Brien went into the woman's workplace and sexually touched her three times without her consent.

At the last hearing, a prosecutor labelled the offending "brazen in nature" given it occurred at a public business.

In a victim impact statement, the woman said as a result of the offending she quit her job and had left the industry entirely, saying she struggled to find peace in her everyday life.

The teenager described being left feeling scared, anxious and confused and said she had suffered significantly in her daily life.

During both offences, O'Brien was living with his parents, had been using the drug ice since 2012 and ramped up his use after he was stabbed for being a "sex pest".

O'Brien has served 10 months of the sentence on remand.