The former Irish and British champion jockey has accepted the invitation of trainer Darren Weir and the Australian Turf Club to partner the ex-Japanese speedster.
"The trainer was in touch and asked would he be keen to go over and he obviously was," said Spencer's agent Niall Hannity.
"He's been looking at the horse's recent races and has spoken to the trainer, who obviously has got an exceptional record.
"It's a very valuable race and he's looking forward to taking part in it."
Spencer's opportunity came when Damian Lane, who has ridden Brave Smash in all his three starts since being brought in from Japan by syndicate owners Australian Bloodstock, opted to remain in Melbourne for the Caulfield Guineas on the same day - rather than go to Sydney.