The Midsummer Sun cobalt inquiry has been put back two weeks at the request of two people charged in the affair.

Vet Dr Tom Brennan and harness racing identity John Camilleri both asked for the delay via legal counsel.

Racing NSW stewards have now rescheduled the opening day of the inquiry to Monday August 3 and it will run for four days.

Trainer Sam Kavanagh faces a total of 24 charges over the elevated cobalt and caffeine levels returned by Midsummer Sun after his Gosford Cup win in January.

He has named Brennan, a partner in the Flemington Equine Clinic, as the person who supplied him with the substance "Vitamin Complex" later analysed and found to contain cobalt.

Brennan has also been charged in Victoria in the cases against trainers Danny O'Brien and Mark Kavanagh, the father of Sam.

On the first two days of the NSW inquiry, Sam Kavanagh will answer eight charges while Brennan and Flemington Equine Clinic general manager Aaron Corby will also answer charges.

On Day Three, another 16 charges against Kavanagh will be heard including race day treatment to Midsummer Sun.

Camilleri, Mitchell Butterfield and Kavanagh's stablehand Michael O'Loughlin all face charges over the raceday drenching of the horse.

Newcastle trainer Darren Smith, the only person so far penalised for cobalt use in NSW, has his second-level appeal against a 15-year disqualification heard on Monday.

Smith failed in front of the Racing NSW Appeal Panel and took his case to the Racing Appeals Tribunal.

Counsel for Smith has argued that because there was no threshold in place at the time Smith admitted to giving his horses cobalt in May last year, he should not be penalised.

Stewards have countered by saying the rules define cobalt as a prohibited substance because it alters a horse's blood pattern.

Judge David Amati reserved his decision on Monday.

A national cobalt threshold of 200mcg per litre of urine came into force on January 1.