An Irish horse who is about to become an Australian has claimed a breathtaking win in the $3 million Cox Plate.

His win follows that of Japanese horse Admire Rakti in the Caulfield Cup, underscoring the quality of the international visitors.

Adelaide came to Moonee Valley on Saturday for the first time but the tricky track held no fears for him or his jockey Ryan Moore who was also a first-time visitor.

And it was a first Australian win for champion trainer Aidan O'Brien who has conquered the rest of the world over the past two decades.

With The Cleaner crossing over to take up the lead as expected, Adelaide ($8) was last coming to the home turn in the unique 2040-metre race.

Moore steered Adelaide wide in the home straight and the horse mowed down the favourite Fawkner ($4.40) to beat him by a short neck with Silent Achiever ($26) another half head third.

Adelaide will join the Chris Waller stable but for now all the accolades are with O'Brien on the other side of the world.

"It has been a wonderful feat," Coolmore's Australian racing manager James Bester said.

"He has been trained from Ireland with Aidan directing everything to the minutest detail.

"He had the best trainer in the world and the best jockey in the world."

Moore, who earlier steered the Coolmore-owned, Waller-trained Bull Point to second in the Crystal Mile, gave up the chance to win the British premiership to chase wins around the world including the Secretariat Stakes in Chicago with Adelaide.

"Things were against him and he did it the hard way," Moore said.

"This horse has shown what a good colt he is. He's been around the world all year and has come down to Australia to round it off."

Classed as a four-year-old in southern hemisphere time, Adelaide gave the others in his age group 6.5kg but Almalad, Sweynesse and Wandjina all finished well back.

The best of Waller's three runners was Foreteller who finished fifth, just behind English visitor Side Glance.

Fawkner's second was another frustration for his owner Lloyd Williams who will have to wait another year to win his first Cox Plate.

John Ingham, who is a partner in the group which bought Adelaide to race in Australia, said it was a dream come true.

"We won a Cox Plate with Octagonal but and it's been a long time for another one," he said.

Roger James was thrilled with the performance of Silent Achiever, a dual Group One winner in Sydney during the autumn who had been unplaced in her previous three runs this campaign.

"I'm proud of her," James said.

"It was a great ride by Nash Rawiller. She was just beaten by two better horses and barrier 11 didn't help her."

Although Adelaide will stay in Australia, his travelling days are not over.

"We have a plan in place," Bester said.

"He will have a spell and then be aimed at the Queen Elizabeth Stakes at the Championships in the Sydney autumn.

"Then we would like to take him to Royal Ascot. He is a wonderful traveller. This is the fifth country he has raced in."