Top jockeys Craig Williams, Luke Currie and Nash Rawiller were stood down from riding at the Flemington meeting by Racing Victoria stewards after being in Sydney during the week.
They have been told to self-isolate at home pending the results of COVID-19 tests.
Fellow jockey Jake Noonan was cleared to ride on the meeting after returning a negative test result.
Craig Williams was booked to ride early Flemington winners Zou Dancer and Long Arm.