An allergic reaction to a shampoo has been unveiled as the culprit that almost derailed Criterion's trip to Hong Kong.

Australian Quarantine and Inspection Service vets didn't want Criterion to fly on Monday after it was thought he had suffered a bout of colic.

They eventually relented and Criterion joined top sprinter Buffering on the plane to Hong Kong.

It was only after his arrival that it became clear the horse had actually suffered a rare allergic reaction after being treated with a regulation tick shampoo prior to his export.

"What he was doing was showing colic-like symptoms because it was stinging, so he was biting at himself and kicking because he was so uncomfortable," co-trainer David Hayes said.

"The vet was puzzled by the amount of pain he was in but there was no stomach noise as you would expect with colic.

"The next day (Monday) he was back to normal but we had a hard time convincing the (AQIS) vet, even though he could see he was healthy.

"It was only when he got to Hong Kong that a rash broke out on his back."

Criterion was placed under saddle for the first time at Sha Tin on Thursday and Hayes said the horse showed no sign of the problem.

He will do light work in the lead-up to the Hong Kong International Cup at Sha Tin on December 14.

Hayes leaves for Hong Kong on Saturday night after he oversees a small team at the Gold Coast on Saturday.