A veterinary examination has failed to shed any light on Southern Speed's below par performance in the Turnbull Stakes and the mare will continue the campaign to defend her Caulfield Cup title.

Trainer Leon Macdonald acknowledged her unplaced run as the worst of her career but said Southern Speed was in good shape on Sunday.

The mare beat just one runner home in Saturday's Group One over 2000 metres at Flemington won by Green Moon, runner-up in the 2011 Caulfield Cup.

"The vet's gone over her and can find nothing wrong," Macdonald said.

"She ate up well last night.

"The track was firm and she may have jarred slightly but that's not an excuse for what was by far her worst ever run.

"We'll press on regardless. It's onwards and upwards and we'll see if I can turn her around."

Southern Speed has won two of her five starts this spring including the Group Two Makybe Diva Stakes where she claimed a nose victory over Manighar who beat her by the same margin in the Australian Cup in the autumn.

While Southern Speed remains a puzzle, Macdonald had some cheer at his home track on Saturday when Viking Star won an open three-year-old race at Morphettville.

His win was part of a treble for his dam Dane Belltar, with the Lloyd Williams-owned Tanby and Fawkner winning stakes races at Flemington.

Viking Star might not be ready for feature races this spring but has won three of his five starts so far.

"I don't think he will be going to Melbourne this spring but he has the makings of a nice horse," Macdonald said.

Macdonald trained Dane Belltar to four wins and placings in both the Victorian and South Australian Oaks.

"She was a good filly and I tried to buy the other two but Lloyd Williams outbid me," he said.

"I don't think it would ever have been done before that three half-brothers could win on the same day.

"She has had a foal by Starcraft this season so that's another one to look forward to."

AAP TURF