Mick Kent is surprised McNulty is still a maiden but the trainer hopes that won't be the case for too much longer.

The three-year-old stayer will be out to breakthrough in Thursday's Listed $150,000 VRC St Leger at Flemington.

McNulty has been slow to pick up on what racing is all about but Kent said "the penny has to drop sooner or later".

A last-start fourth in the Listed Galilee Series Final (2500m) at Flemington, McNulty is expected to handle the step up to 2800m in the St Leger.

"He's always appealed to me as a nice sort of stayer but he's been a bit slow picking up on what to do and has needed racing," Kent said.

The St Leger has been the main aim for McNulty this campaign although Kent hasn't ruled out extending his preparation to include the South Australian Derby (2500m) on Saturday week.

"But he'd have to come through tomorrow and be impressive and please me," the trainer said.

McNulty was unplaced in three starts last campaign and then returned with placings at Cranbourne and Bendigo as favourite in maiden company.

In his most recent start in the Galilee Series Final, the stayer settled back in the field and was outsprinted by the winner Shoreham but kept working home fairly for fourth.

Kent said the way McNulty came through the run was a sign he had more to offer.

"He's a very big, tough horse and that's the thing about him, you wouldn't think he had a run the other day," Kent said.

"I think that's because he hasn't been putting it all together yet.

"I've never got to the bottom of him yet."

The Gai Waterhouse-trained Hippopus looms as the obvious horse to beat in the St Leger on the back of his Sydney autumn carnival form.

Kent is aware of the Waterhouse-trained galloper's ability as Hippopus was second to the Kent-trained Philippi in the Group Two Tulloch Stakes (2000m) before his fifth in the Australian Derby (2400m) where Philippi was second.

Hippopus, one of two Waterhouse-trained St Leger runners, is favourite ahead of Serpent with McNulty on the fourth line of betting.