Korean cleaners have been paid illegal cash wages of $15 an hour, which an employer has attempted to argue is the "going rate" in Australia's Korean community.

Two Korean cleaning workers, who are on working holiday visas, are believed to be owed almost $10,000 for just three months' work at a Sydney college between March and June this year.

Investigations found the underpaid workers were receiving illegal flat rates.

The troubling revelations have come to light after exploited cleaners lodged complaints with the national workplace watchdog.

When questioned about the illegal pay rates, the Sydney cleaning contractor told officials that $15 an hour was considered common pay among Korean businesses.