Miss Federer took advantage of a good run in front, set up by rider Kylie Williams, to bounce back to her best form in the $50,000 race.
Carston admitted he had not enjoyed a smooth path into the race with Miss Federer.
The horseman consulted some of New Zealand’s top trainers, including Stephen McKee and Lisa Latta, as he battled to get the filly back to her best for the 1400m race.
"We’ve had a bit of a tough time with her of late so it is very satisfying to see her win this one," Carston said.
"They have just been niggling problems and she wasn’t breathing that well in her races.
"I was asking around some other trainers and got plenty of good advice that we have managed to put into practice, so I’m delighted we could get the win."
Miss Federer showed she was in the frame of mind Carston wanted her to be in when she relaxed beautifully after Williams pushed her to the lead early.
"She didn’t get a lot of pressure from the others and when she got some cheap sectionals, I knew she was going to be hard to beat," Carston said.
There was no chance for Miss Federer to relax at any stage of last month’s Gore Guineas.
The filly was savaged in the lead by Burgundy Rose, who dropped out to run 11th.
Miss Federer fought on bravely and finished just over two and a-half lengths from the winner, Lightning Jack, in eighth.
Carston then raced the 3yr-old at her home track of Riccarton two weeks ago where she ran a solid second behind Live Drama, who was seventh on Saturday.
"I thought it was a good effort by Kylie after I was critical of the ride on her at Gore last time," he said.
"She took that on the chin like the professional she is and came out today and produced a copybook performance."
Runner-up All About Magic tried hard in the straight to get within three-quarters of a length of Miss Federer at the finish.
The Michael and Matthew Pitman-trained filly will not have to contend with her rival in the next leg of the South Island Filly of the Year series.
Carston confirmed Miss Federer would not attempt to run over 1600m in the next listed race at Riccarton on April 4.
The trainer admitted he was unsure about where to aim her following her victory and would take some time to consider it.
All About Magic was trailed in by the favourite, Trifolium.
She was ridden into third by Samantha Wynne, who replaced Chris Johnson, who was stood down after suffering from dehydration after race 3.
Earlier on Saturday, Michael Pitman notched his 1800th win in New Zealand when Tee Cee Coup won race 3.
The Swiss Ace 4yr-old produced a late finish to hand her trainer the milestone victory in rating 72 company over 1200m.
Pitman said during his battle with bowel cancer over the past 18 months he was not sure he would achieve 1800 wins
"Twelve months ago I wasn’t sure I would be here, but I have got an all-clear. I am as ambitious as I have ever been."
