Diamond Effort has completed a winning hat-trick and at the same time added black-type to her name with a narrow victory at Mornington.

Following successive Caulfield victories to start the campaign, Diamond Effort added Saturday's Listed Hareeba Stakes to her record, a sixth win from 12 starts.

Ridden by Ben Melham, Diamond Effort was sent out the $4.40 favourite before claiming Runson ($7) close to home to score by a short neck with Seabrook ($19) a half length away third.

Trainer Clinton McDonald took advantage of giving Diamond Effort experience at Mornington on Tuesday although the Caulfield-based horseman was concerned the mare may not have worked hard enough while Melham thought she was spot-on.

"I worked her on Tuesday and Clinton was worried I might not have done enough with her," Melham said.

"I said 'don't worry about it mate, it'll win'.

"I was right but he was right as well because she was pretty fresh.

"She just fought me a bit in the run and did a good job to win, I thought."

McDonald said Diamond Effort had been set for Saturday's race and may now be aimed at the Listed Wangoom Handicap at Warrnambool during the May three-day carnival.

But the trainer said he could also raise the bar and look at the Adelaide carnival and the Group One Sangster Stakes at Morphettville on May 2, four days before the Wangoom.

"They're all races we can have a look at it," McDonald said.

"Obviously we're going day by day in racing at the moment so we'll take what we can get.

"The main thing with her is we just give her plenty of space between runs and she seems to enjoy that.

"If the race is there we'll have a look at it."