A key left-wing union is preparing to confront Victorian Premier Daniel Andrews at Saturday's Labor Party state conference and demand sweeping reform to protect school cleaners from "wage theft".

The state secretary of United Voice, Jess Walsh, will move an "urgency resolution" at the party's conference at Melbourne's Moonee Valley Racecourse, and unveil the results of an audit suggesting a staggering 80 per cent of school cleaners are being paid below-award rates.

According to the results of the audit, in which officials visited 142 schools, cleaners are routinely hired on sham ABN contracts or on an off-the-books basis, and the average underpayment was estimated to be $3000 a year.

School cleaners plan to approach Mr Andrews when he enters the conference.