Unforgotten's uneasy start to the autumn is now a distant memory.

Diagnosed with cardiac arrhythmia after running last in the Group One Surround Stakes last month, Chris Waller never lost faith in the three-year-old filly.

Sydney's premier trainer was also unfazed when Hiyaam beat her in the Group One Vinery Stud Stakes (2000m) and a fortnight later Unforgotten turned the tables in style at Randwick to claim the $1 million ATC Australian Oaks (2400m) in record time.

The $4 favourite levelled up to front runner Hiyaam at the 100m and cruised home by a length in 2:27.21, to eclipse Serenade Rose's 2006 mark of 2.28:80.

"She had the perfect run the way the race was run. We knew Hiyaam was the one to chase, she set about chasing and it was go to see the upper hand," Waller said.

"The first question was her stamina and whether it would hold up.

For a three-year-old filly it's quite a long way, you don't know about 2400 metres until you run it."

Waller, who won the Australian Oaks with Royal Descent in 2013, said he also comforted by Unforgotten having Galileo as her damsire.

"In Europe you've got Galileo and in Asia there's Deep Impact, there's no better staying blood than those two stallions at the moment," he said.

Hugh Bowman bided his time and has now won five Australian Oaks, and the last three in a row after previously saluting on New Zealand raiders Sofia Rose and Bonneval.

Waller reckoned he had an Oaks quality filly around Christmas before she struggled in the Surround Stakes.

"They can get a bit anxious first-up, that's what I was clinging to."

Unforgotten bounced back in the Group Two Phar Lap Stakes (1500m) on March 17 and made steady progress to this staying test.

Waller was looking forward to Unforgotten developing and insisted she would be given time to mature.

"We won't be rushing to step her up to a Caulfield Cup or a Melbourne Cup," he said.

"We'll keep our sights low and see how she comes back as a four-year-old," he said.