A mare who never runs a bad race, a country trainer who is among the top four in the nation and a jockey who has acquired a Group One habit have won South Australia's premier sprint.

Platelet ($9) added the $500,000 Goodwood to her success two weeks ago in the Robert Sangster Stakes, the same double won last year by Black Caviar, racing away to a three-quarter length win over local runner Conservatorium ($71) with Altar ($13) a short-neck third.

For Ballarat-based trainer Darren Weir, who last year finished third on the national table, the victory provided further confirmation of his standing among the best in the game.

And for jockey Ben Melham it gave him his third Group One win in as many weeks.

Melham rode Platelet to victory in the Sangster and last Saturday won the South Australian Derby on Escado.

Melham deflected praise to Weir whose training he described as "unbelievable".

"This mare has been at her top for two seasons and this week she was probably as good as she has ever been," Melham said.

"Darren Weir is one of the best there is and he's shown it today."

One of the best to begin, Platelet held a three-wide position just behind the leading pack until entering the straight when Melham moved wider to gain clear running.

After balancing up, Platelet hit top gear at the 100m, pouncing on the leaders and drawing away for a convincing victory that took her racetrack earnings to some $950,000 from a record of seven wins and 10 placings from 19 starts.