For a horse set to start the shortest-priced Australian Derby favourite in 25 years, It's A Dundeel has his share of detractors.

His failure as favourite in the Victoria Derby last spring cast doubts over his stamina, leading some rival trainers and jockeys to wonder whether he might be vulnerable in a true staying test.

But Nash Rawiller, who will partner Hippopus for Gai Waterhouse, has learned the hard way not to underrate the New Zealander.

"I probably underestimated him when he beat me (on Proisir) in the Spring Champion but since that day I've taken notice," Rawiller said.

"But he has gone to a whole new level this preparation."

It's A Dundeel has claimed the Randwick Guineas and Rosehill Guineas at his past two starts, the latter in spectacular fashion.

But still, a potential vulnerability over the longer 2400m Derby trip nags.

Rawiller might well prove best placed to expose any chink in the favourite's armour.

Hippopus looks the natural leader and while Rawiller says he will simply ride the race to suit his mount, his tactics could prove crucial.

"This bloke's best asset is that he can stay," Rawiller said of Hippopus.

"I'm not saying the other bloke (It's A Dundeel) can't, but I've got to ride my horse to best suit him.

"We will see what happens from there."

Hippopus comes into the Derby off the back of successive 1900m wins at Canterbury and a last-start second to Derby rival Philippi in the Tulloch Stakes.

The Tulloch was Rawiller's first race ride on the three-year-old who he believes is still improving.

"With every race start the penny has dropped that bit more," Rawiller said.

"The other day (in the Tulloch) he was held up at a crucial stage and I'm not saying it cost him the race, but it cost him his momentum turning for home.

"He's the sort of horse where that takes a bit off him, he's a free-roller."

The Derby lost some of its lustre earlier in the week when the Anthony Cummings-trained second elect Fiveandahalfstar was ruled out due to injury.

It left a field of 10, including Corey Brown's mount Gerontius who finished fifth in the Tulloch after getting back to last from a wide gate.

"I didn't think his last run was too bad, especially the last bit. He ran through the line really well," Brown said.

"It was a two-horse race then Cummings's horse came out and now it looks a one-horse race. But you never know."