A spike in a pre-race bi-carb test and the subsequent betting ring support for Flemington runner-up The New Boy prompted the temporary impounding of both him and winning stablemate Kutchinsky after Saturday's meeting.

Racing Victoria chief steward Terry Bailey said on Sunday the pre-race screen indicated a jump in the TCO2 levels of The New Boy, but he emphasised they had not reached the prohibited mark.

The stewards held the New Boy and Kutchinsky at Flemington for some time after racing and then, along with vets, escorted them to the stables of their trainer Danny O'Brien and remained with them until further samples were taken.

Bailey told RSN radio in Melbourne the procedure is part of Racing Victoria's drug testing strategies which are to be ramped up from March 1.

He explained that a horse's TCO2 levels will normally drop as a result of the exertions of a race.

But if the horse has been treated within the prohibited 24-hour period before the race, the levels will generally rise within three hours after racing.

"When we got elevated levels, we thought there's enough there to go through the process," he said.

"Not that The New Boy was anything excessive.

"There was plenty of money for The New Boy to suggest it could win and that was the reason why it was done.

"There have been cases of horses returning positive swabs some time after they've raced."

Stewards took blood samples from both horses, the results of which are expected to be available by Wednesday.

O'Brien applauded the action saying the stewards had earned kudos for the stand they are taking against drugs.

"Kudos to Terry Bailey and his team ramping up the testing regime," O'Brien said in a tweet.

"Very happy to seem them target testing winning stables. Keep it up."

Bailey said there would be a lot more similar testing performed after the introduction of new rules on March 1.