Star jockey James McDonald will spend 18 months on the sidelines after he pleaded guilty to having an interest in a bet on Astern in December last year.

McDonald appeared before the Racing NSW stewards on Thursday where AAP reported "his lawyer Matthew Stirling argued he should be given a two to three month ban and fined”. "The betting public including the owner and the trainer would think 'well he is trying'," Stirling told the inquiry.

But stewards took sterner action disqualifying the rider for 18 months. His ban was backdated to start on November 15 when McDonald stood himself down from riding after being questioned over the indiscretion by stewards. McDonald will be allowed to return to race riding on May 15, 2018.

McDonald won $4000 from a $1000 bet placed on his behalf by punter Anthony Gardiner. The charge carries a mandatory two-year disqualification but Stirling argued because McDonald rode the horse, the integrity of the race had not been compromised.

Gardiner, who has not responded to stewards' requests to be interviewed over the matter, profited in excess of $125,000 from bets on Astern from the race. "There is no doubt that Gardiner manipulated the relationship," Stirling said. "McDonald's good character and remorse over the bets should be considered.”

Stewards had previously warned McDonald about his association with Gardiner before the Tramway Stakes on September 3. Gardiner backed rival horse Dibayani who was competing against McDonald's mount Hauraki in the Group II race at Randwick. Stewards saw McDonald and Gardiner acknowledging each other in the mounting yard and as the horses went on to the track.

Chief steward Marc Van Gestel spoke to McDonald behind the barriers, warning him he would be watched closely during the race. Hauraki won by three-quarters of a length while Dibayani finished third.