A Waikato racehorse has staked his claim to being among the world's elite gallopers after winning the A$3 million (NZ$3.8m) Cox Plate at Melbourne's Moonee Valley on Saturday.

Bred by Cambridge's Brent Taylor and trained by Matamata part-owner Gary Hennessy, Ocean Park produced a performance of rare quality to win the Group I 2040m feature dubbed Australasia's weight-for-age championship.

No New Zealand-trained horse had won the race since champion mare Sunline went back-to-back in 1999 and 2000.

"We set a plan way back in the autumn and today was huge. It's great. I'm really pleased for all of New Zealand," Hennessy said.

"People weren't sure about him but I was. He goes out and does what he has to do," he said.

A 4-year-old stallion by Windsor Park Stud sire Thorn Park, Ocean Park is set to race on, with Hennessy keen to defend his title in next year's Cox Plate before the horse embarks on a stud career.

Saturday's win would have increased Ocean Park's stud value to more than $10m. However, it has not yet been decided where he will spend his days as a stallion.

Ocean Park was ridden to victory by Melbourne jockey Glen Boss, whose big wins include riding Makybe Diva to three successive Melbourne Cups.

"'It feels like my greatest achievement in this sport so far," Boss said.

"I know I've done some wonderful things but I've been in this colt's corner the whole time and there's been doubters," he said.

"I kept saying he is a winner.

"He just wants to win, he just wants to beat whatever is in front of him."

It was Ocean Park's fourth straight Group I weight-for-age win, taking his record to seven wins from 11 starts and his earnings to more than $3.27m.

He has won more than $3m in the past two months.

It was a memorable day for Waikato racehorses at Moonee Valley.

Cambridge mare Silent Achiever, trained by Roger James, won the Group II A$225,000 Crystal Mile for Australian-based Waikato jockey James McDonald, while the Murray Baker-trained Dundeel (It's A Dundeel across the Tasman) finished second in the A$225,000 Mitchelton Wines Vase, with the Victoria Derby the main target.

- © Fairfax NZ News