It began with an opening bid of $2 million and ended three minutes later with a successful offer of $5 million and the announcement that history had been made.

But the most expensive thoroughbred yearling ever sold in Australia almost didn't make it into the ring.

The colt, a half brother to the world champion sprinter Black Caviar, by the champion stallion Redoute's Choice, threw a substantial tantrum on sale eve that could have resulted in a serious injury and led to a decoy horse being placed in his box.

A member of the team from Three Bridges Farm who prepared the colt for the sale said he became agitated after constantly being removed from his box to be inspected.

"He just cracked it, he got jack of being taken in and out of his box and lost it," said the Three Bridges representative.

"If it hadn't been for the skill of his handlers it could have been a disastrous."

In a bid to avoid the inevitable attention on sale day, inspections were banned and the colt was shifted to a nearby box and a substitute yearling stood in his place, attracting the appreciation of the hundreds of unwitting admirers.

On the day though, the record-breaking colt was on his best behaviour as he walked five laps of the sale ring, prompting a bidding war between Victoria's Swettenham Stud and the international buying group BC3 Thoroughbreds.

The group's Australian chief Bill Vlahos eventually survived, splurging $5 million with a nod of his head.

Vlahos also bought last year's sales topper, a $2.6 million filly who, like the $5 million colt, is also out of Black Caviar's mother Helsinge.

"I want to keep the blood here in Australia," he said.

"The filly last year nearly went to Japan. We want the purest of Australian blood to stay in Australia.

Registered as Belle Couture, she is yet to race.

While the risks in buying any yearling are substantial, the Redoute's Choice colt has a better chance than most of returning a profit.

Apart from Black Caviar, the broodmare Helsinge has produced the star three-year-old All Too Hard and the talented and now retired Moshe.

"Belle Couture will be winning a nice race before this colt ever races. His value is safe," Vlahos said.

"If this horse wins a Group One race his owners are probably sitting on $40-to-60 million."

Vlahos is yet to choose a trainer for the horse or to finalise its ownership.

But he said the colt would got to a Victorian trainer, possibly Black Caviar's trainer Peter Moody or John Hawkes who trains All Too Hard from his Melbourne stable.

From the opening bid of $2 million, the action stalled at $2.5 million for several seconds before Vlahos entered the fray.

He and the under-bidder went bid-for-bid in $100,000 increments with the 15th bid securing the colt.

The previous record for a yearling in Australia stood at $3 million, an amount paid twice previously for horses that have been notable only for their lack of ability.

As is the world's most expensive yearling, The Green Monkey, a $16 million purchase in Kentucky who retired winless after only three starts.