Carston rode Joint Effort to win the first of six races in the series but the points she gained were credited to Tomohiro Sasada, who had been allotted the mount.
Sasada, Tyler Johnson and Natasha Collett missed the first race when the arrival of North Island apprentices was delayed by the closure of Wellington airport to some flights.
Carston clinched the series when she rode Crowded House to finish third in the final race.
She had been sharing the lead with Natasha Collett and Craig Grylls leading into the race.
Carston recorded her win under difficulties.
She was struck in the right eye by a clod early in the race.
"It was a bit blurry for the whole race," she said.
Carston (26) was the oldest contestant.
She has been apprenticed the past 18 months to Shane Kennedy at Riccarton.
Her tally is 30 wins including 21 this season.
She had not ridden a horse until she was 21.
She had been in other employment when she visited her brother, Andrew, at Matamata, where he is in employed at the Mark Walker stable.
She began riding work for Jon Sargent and started her apprenticeship with that trainer.
She rode her first winner, Band Of Colours, at Kumara in January last year.
Carston collected a $1000 cash prize and a trophy for her success.
Natasha Collett and Craig Grylls split $750 when they finished second equal with 13 points.
• Ashlee Mundy, who finished fourth, achieved her 100th win when she rode Burgundy Dancer.
Mundy rode Starkay to win at Oamaru in August, 2004 when having her first ride.
Mundy (21) said she intended to apply for a full licence after she rode her next winner, thus losing her apprentice claim.
She extended her apprenticeship in March after serving four years with Michael Pitman.
• John McKay collected a $1000 prize for the most successful trainer in the apprentice series.
He had wins with Burgundy Dancer, Rollick (James McDonald) and Bilbo Baggins (Jay Misbah).
Rollick joined the Wyndham trainer last month. McKay has been been based at Oamaru for the past three weeks.
Paul Harris collected $500 for his second placing after wins with Joint Effort and Devious Diva (Tyler Johnson).
Harris said Devious Diva would race at Oamaru next month in a bid to guarantee a start in the Winter Cup at Riccarton.
