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Yamamoto won on Ambaarun, trained by Pam Gerard at Ashburton.
Yamamoto (25), who has ridden 149 winners in Japan, has been with the stable for a month.
She began work with the stable of Andrew Scott at Matamata when she came to NZ 12 months ago.
Yamamoto had her first NZ ride at Riccarton on Saturday. She had four rides yesterday.
Yamamoto brought Ambaarun along the inner from last to win by a head from Caparison.
Ambaarun had been placed in her two other starts since transferring to Gerard six weeks ago from Brett McDonald at Cambridge.
The 6yr-old mare is owned by McDonald's wife, Di, and John Bishop.
Yamamoto was suspended yesterday for careless riding, but stable representative, Tom Hazlett played that down.
"She [Yamamoto] is just rusty and Waikouaiti is not the ideal track for her to kick off again," Hazlett said.
"I am delighted that she got a win. She is an exceptionally good worker and rides Ambaarun in training."
Yamamoto was suspended for five riding days until midnight on April 23 for shifting in early in race two when riding Come On Jimmy who finished third. She caused tightening to Our Mona Lisa.
Biman mastered top weight of 58kg to win the Publicans' Cup yesterday.
He is trained at Tapanui by Tania Booth, who races the Colombia gelding with her father, Graham Young, an Auckland farrier.
Biman outfinished King Sapphire by half a length.
Jan Cameron was charged with failing to ride out Mortification who finished fourth in race three.
She will also be charged with leaving the course before the charge could be heard.
Jason Laking, who rode Pentiful to win, was suspended from April 11 to 25 inclusive for allowing his mount to shift in over the closing stages, causing a check to Magic Trick, who finished third.