Champion jockey Damien Oliver has been suspended for almost a month after failing a breath test during Warrnambool's famous Grand Annual meeting.

Oliver's Racing Victoria stewards' ban will start immediately, ruling him out of riding at Morphettville on Saturday.

He had five rides in Adelaide, including bookings in two Group One races - Asinara in the Australasian Oaks and Vezalay in the Robert Sangster Stakes.

He can return to riding for the Queensland Oaks meeting on June 4.

Jockeys were breath tested at the Warrnambool course on Thursday and Oliver had a reading of 0.038 when first tested before returning 0.035 in a second reading.

Oliver, who pleaded guilty to a charge of being over the 0.020 limit for jockeys, said he went out on Wednesday night for a few drinks before getting home at midnight.

He said he departed from his usual race-morning routine of having a sauna.

"I'm very disappointed and embarrassed," Oliver told stewards.

"I've never had any problems with this rule before in 20 to 30 years of riding."

Jumps jockey Paul Hamblin was suspended for a month for failing a breath test which resulted in Shiny Manners being scratched from the Novice Hurdle when there was no other rider available.

Hamblin blamed a mouth wash for his higher than allowed reading.

His first reading was 0.033 with a second test 16 minutes later showing a 0.022 reading.

Hamblin told stewards he had consumed five glasses of rose on Wednesday night but had a small meal for dinner and he had skipped breakfast.

He can return to riding on June 5.