Plonka has given trainer Warren Gavenlock his biggest win and booked his ticket to the $500,000 Country Championship final with his heat win at Grafton.

The Northern Rivers $150,000 qualifying heat was run on Thursday after being delayed then transferred from a waterlogged Coffs Harbour.

Under confident riding from Rachel Murray, Plonka beat the Matt Dunn-trained Gracie Belle to win the 1420m-race with the runner-up also booking her trip to the Sydney final at Randwick on April 4

Plonka had already given Gavenlock a milestone city win in a Highway Handicap at Rosehill in November.

"A lot of people doubted Plonka could run the 1400 metres but I never did and what a great ride," Gavenlock told Sky Thoroughbred Central.

"The delay of four days played in our favour and the drying track helped as well.

"He was my first city winner and he'll give the final a shake."

Murray said she put her faith in Plonka and let him work into the race in his comfort zone.

"It was a huge effort," Murray said. "I am not going to lie, the extension of the qualifier out to Thursday, it gave him another four days to recover.

"Everything worked out perfect. He is a good horse.

"I just love the horse and I just have so much faith in him so I didn't care where I was. I just did my own thing."

Gracie Belle became somewhat agitated pre-race and did well to run second.

"She got herself a little bit upset in the yard and it probably followed through to the race," Dunn said.

"I'm disappointed we didn't win but full credit to the winner."

Plonka and Gracie Belle join Sunday's South East regional qualifiers, Julian Rock and Al Mah Haha in the final.