A disqualified harness racing trainer says he drenched and injected the Sam Kavanagh-trained Midsummer Sun hours before his Gosford Gold Cup win.

A post-race swab taken from Midsummer Sun after his January win showed an illegal level of cobalt in the stayer's system and also tested positive for caffeine.

Kavanagh was stood down from training after making certain admissions to stewards about treatments his horses received, prompting a Racing NSW stewards' investigation that has extended to Victoria.

The trainer was at the Gold Coast on January 9, the day Midsummer Sun won at Gosford, but was aware the horse was to be treated.

Mitchell Butterfield gave a statement to stewards, saying John Camilleri, another harness racing identity, was originally going to treat Midsummer Sun before the Gosford Gold Cup.

But at the last minute, Camilleri could not make it and asked Butterfield to go to Rosehill where he said he drenched and injected the horse.

Neither of two stablehands interviewed on Thursday said they saw any strange people at the stable.

A group text message extracted from Camilleri's mobile telephone by stewards said: "We like Gosford R6 3. It's had my magic".

Kavanagh spoke to Thursday's hearing by telephone and said his interpretation of the message was Midsummer Sun had been treated as instructed.

"I was frustrated and desperate and I trusted the wrong people," Kavanagh said.

"I stopped when my partner said she and my daughter would leave if I kept letting John Camilleri treat my horses."

Alan Thompson, a part-owner of Midsummer Sun and other horses in the stable, said he and Kavanagh had discussed using xenon gas on the horses.

Thompson, who works for a pharmaceutical company and is a personal trainer, said he was unaware it could be harmful.

Kavanagh said he only used the gas, which has subsequently been banned, on a horse which had an injury.

Thompson said he and Kavanagh would often bet on each others' behalf but in amounts of no more than $500.

Dr Tom Brennan, a partner in the Flemington Equine Clinic, who has been charged in Victoria in relation to cobalt offences, has been named by Kavanagh as the source of the substance called Vitamin Complex he used on his horses.

Another Flemington Equine Clinic vet, Dr Stuart Vallance, confirmed on Thursday, he had seen a bottle with the same label in the portable fridge in Brennan's car.

Analysis showed it had a high concentration of cobalt.

Stewards will now determine whether Kavanagh should face further charges in addition to the eight already issued.

The inquiry will resume no sooner than June 29.