After initially being turned down by Racing Victoria's apprentice school, 21-year-old Dylan Dunn is about to be crowned Melbourne's champion junior rider.

Dunn claimed the title on Wednesday morning when his only challenger, Michael Dee, failed in an appeal to have a careless riding suspension overturned.

Dee ends the season at least two wins adrift of Dunn who rides at Caulfield on Saturday.

"It's massive for me," Dunn said on Wednesday before being stood down after one ride at Sandown because of dehydration.

"I never expected to be good enough calibre to really compete against those other younger jocks (for the premiership), just for the fact that I can't ride 52 kilos and I have a minimum of 55 kilos.

"I've tried to be smart and keep it at that one level and it's paid off for me."

Dunn recently said he had never wasted so hard to ride at 55kg each week in his pursuit of the premiership.

His weight was the reason he was first rejected as a Racing Victoria apprentice and he went to South Australia to start his career in the saddle.

He started his apprenticeship with Phillip Stokes in Adelaide before making the move back to Victoria during the 2014/15 season.

"It was obviously a little bit disappointing when I worked so hard to get my weight down from 67 kilos, playing footy, and then getting turned down," Dunn said.

"South Australia have been amazing to me and they gave me the opportunity and then Racing Victoria was good enough to say I had earned the right to come back.

"Since I've been back they've been supporting me, and I think hopefully this just proves that I am good enough to compete over here and I can try to outride my claim now."

Dunn said outriding his 3kg city claim had been his aim for this season, something he achieved in January before taking a short holiday.

Around that time the apprentices' premiership became a focus.

"I came back (from the holiday), got on the right horses and was refreshed in the mind and hit a bit of a purple patch," he said.

"And that really spurred me on to go for the premiership."

Dunn said the battle with Dee for the premiership had been fantastic.

"He's an amazing rider on the up and I'm trying my hardest too," Dunn said.

"We've been spurring each other on and we're good mates outside of racing."