Royal Descent has posted one of the biggest modern-day winning margins in a Group One race, annihilating her opponents by 10 lengths in a breathtaking Australian Oaks victory.

But it almost didn't happen.

Trainer Chris Waller revealed he came close to scratching the filly after Randwick continued to be battered by rain and the track was downgraded to heavy.

He desperately sought the thoughts of owner Gerry Harvey and his racing manager Claire Bird just hours before the race before deciding to run.

Connections must have been wondering why they ever wavered when the filly posted the biggest winning margin in a Group One race since Might And Power took out the 1998 Queen Elizabeth Stakes by 10-1/2 lengths.

Royal Descent ($4.40) settled worse than midfield for jockey Nash Rawiller but improved so rapidly coming to the corner she had joined the leaders.

She accelerated further in front down the Randwick straight to easily defeat Victorian Oaks winner Dear Demi ($7.50) and Gondokoro ($18), with favourite Habibi ($3.70) struggling home for fourth.

The Australian Oaks (2400m) only became an option for Royal Descent after she won the Adrian Knox Stakes impressively seven days ago, and even then Waller wasn't committed to backing her up.

"Even today I spoke to Claire Bird and Gerry Harvey several times and we made the decision at one o'clock on whether to run," Waller said.

"My only instructions to Nash were `if you're not going to be a winning possibility then don't knock her around too much'."

Rawiller glanced at the in-field screen over the final furlong and when he realised how far in front he was he gesticulated his joy, earning him a $400 fine from stewards.

The jockey was overwhelmed by Royal Descent's performance.

"She's gone beyond all expectations today," Rawiller said.

Dear Demi's rider Jim Cassidy bore the brunt of a spray from trainer Clarry Conners over his effort on Dear Demi but Cassidy said nothing would have changed the result.

"The whole point of taking the blinkers off was to be up closer," Conners said.

Cassidy told Conners "the winner's a swimmer" before they both agreed the wet track went against her.

Habibi's trainer Donna Logan said the filly floundered in the heavy going.

"It was the worst run of her life but she's still run fourth and she deserves a break," Logan said.