Cosmologist will be a surprise starter in the $500,000 Country Championship at Randwick after his win in the qualifying heat in atrocious conditions at Mudgee.

Sent out at $51 in Sunday's heat, Cosmologist went straight to the front with his jockey Eleanor Webster-Hawes unable to see the other runners because of driving rain.

The Dean Mirfin-trained gelding beat another $51 chance, the Gayna Williams-trained Noel's Gift who was last rounding the turn with Grant Buckley steering him up the rail.

Mirfin took the gamble to start Cosmologist, who was an emergency in the $150,000 race, when there were four scratchings.

Cosmologist had also been entered for another race on the program.

"I couldn't see anything," Webster-Hawes told Sky Thoroughbred Racing.

"I just tried to keep the horse comfortable and balanced."

Buckley said he had to abandon his goggles and decided to stick to the fence, using the rail as a guide.

"All I could see was the fence to guide me around," he said.

"I've never ridden in conditions that bad."

Race caller Col Hodges could not see the horses until the final 250 metres and the last horse home finished almost 50 lengths from the winner.

While many trainers were ruing the downpour, Mirfin could not contain his excitement.

"It's unbelievable," he said.

"When Cosmologist emerged from the gloom I thought it can't be, but it is.

"This is by miles my biggest win."

On Saturday, Danny Williams trained the trifecta in the heat at Goulburn with the first two, Kopi Luwak and Pumpkin Pie, automatically in the final.