A forecast for rain in Sydney this week is Gerald Ryan's only concern for boom colt Rubick ahead of his highly anticipated debut at Randwick.

While the youngster has drawn barrier six of seven for Saturday's TAB iPhone App Plate (1000m), it is the weather forecast, rather than the gate, occupying his trainer's mind.

"I don't worry about the draw but I'm worried about the rain that's forecast," Ryan said.

"There is 20 or 30 mils forecast for Friday. I would have to talk to the owners but I wouldn't run him on a heavy track."

Rubick has been heavily backed to win next month's Blue Diamond Stakes and is second favourite for the Group One despite not having raced.

Provided the Randwick course proper remains out of the heavy range, Ryan expects Saturday's race to provide his youngster with a strong test ahead of a trip to Melbourne.

"It's the best two-year-old race this year off the trials," Ryan said.

Saturday's program will also be used as a springboard to black type races for former Victorian mare Saturn Rock who will have her first start for trainer Grahame Begg in the TAB.com.au Handicap (1200m).

The four-year-old has been placed five times in stakes company without winning, a frustrating run her new owners hope Begg can help break this autumn.

"We're hopeful she can pick up some black type before she goes to stud," Begg said.

"Hopefully, we can find the right race. There are a couple of races in March that could suit her, the Birthday Card (Stakes) and those sort of races."

Previously prepared by Robert Smerdon after beginning her career in New Zealand with Shaun Ritchie, Saturn Rock was sold at last year's Magic Millions Broodmare Sale.

Begg has given her two barrier trials in preparation for Saturday and she traditionally races well fresh.

"It's hard to know how she will go but I've watched her runs in Melbourne and she gets back and hits the line well," Begg said.

"She's fit enough."

The Listed Australia Day Cup (2400m) has attracted a final field of eight with Chris Waller and Gai Waterhouse holding two entries apiece.

Waller will saddle up import Brigantin and Summer Cup winner I'm Imposing while Waterhouse will be represented by Travolta and Rule Book.