Chris Waller is known for being meticulous with his horses but he concedes he is at the mercy of genetics when it comes to the 3200-metre stamina test of Saturday's Stayers Cup at Randwick.

The trainer has ensured his trio of runners - Permit, Secessio and Wazn - have a strong fitness base but he doesn't believe a horse can be conditioned to be competitive over 3200m at metropolitan level without the right make-up.

"At this level it comes down to their genes and their ability to run a long distance," Waller said.

Races over the Stayers Cup trip are increasingly rare in Australia despite it being the distance of the country's flagship contest - the Melbourne Cup.

Waller says the dearth of opportunities to test a horse at 3200m makes it difficult to asses the chances of his trio, or those of most rivals.

Permit is the only member of his Stayers Cup team to have attempted the trip - once when he was inexplicably retired from the 2012 Sydney Cup mid-race, and again in last year's Stayers Cup when he finished seventh but was coming off a testing autumn carnival campaign.

"I believe you throw a form guide out the window when it comes to two mile races," Waller said.

"It's changed a little bit in recent years when you're getting imports coming in, especially in a Melbourne Cup where you're getting good form, but apart from that we don't know who wins a two mile race because we only have a couple of them.

"I don't know whether Secessio will stay, I don't know whether Wazn will stay.

"Permit failed as favourite in a Sydney Cup. Did he stay? Did he choke (down)? Did someone shoot him? It was at the 1000m mark (he was pulled up) and we know he goes further than that so he must have choked or something went wrong."

Bookmakers have Secessio the top pick of Waller's runners at $3.50, behind $2.25 favourite Destiny's Kiss for Joe Pride.

Saturday's test has been made all the more gruelling by a lashing of rain at Randwick on Thursday.

The track was rated in the slow range mid-morning but as rain persisted during the day, officials believed it was likely to be further downgraded to heavy.

That won't suit Permit, who is unplaced from four starts on heavy ground, although both Secessio and Wazn have won in the conditions.