Leading trainer Darren Weir has had a horse return a positive test to a banned substance in a jump-out at a country racecourse.

Air Guitar returned a positive swab to phenylbutazone and oxyphenbutazone, a non-steroidal anti-inflammatory at Burrumbeet in June.

The three-year-old was contesting a jump-out for the purpose of gaining a permit to start.

Weir was informed by Racing Victoria stewards of the irregularity on August 3 before they recently advised him of confirmatory findings as they continue their investigations.

Weir was last season's leading Australian trainer, preparing 449 winners, and currently leads this year's tally with 28 winners.

Fellow Ballarat trainer Simon Morrish has had the unraced Miss Astronomoore return a positive swab to the same substances at the same set of Burrumbeet jump-outs.

Like Weir he was informed of the irregularity on August 3 before the confirmatory findings.

Morrish already faces a charges over the scratching of Show A Star at Flemington in July.

Stewards allege Morrish administered an unknown medication to Show A Star, without their permission, on July 8.

He has also been charged with injecting, or causing Show A Star to be injected at some time on the day of the race or in the one clear day before the race and also for failing to record a treatment to Show A Star on July 4.