Santa Monica has caused an upset in the Railway Stakes at Ellerslie in Auckland, the first Australasian Group One race for the year.

Ridden by Trudy Thornton and paying $77 on the New Zealand TAB, the five-year-old nabbed Princess Kereru on the line to win the 1200m-sprint by a neck.

Favourite Melody Belle was another head third after being posted wide throughout.

"She has been knocking on the door and we knew we could win this," Santa Monica's trainer Stephen Ralph said.

"The only horse we feared was Melody Belle.

"It was great work by Trudy. She has stuck with us the whole way through, she knows the horse and just summed it up perfectly."

Thornton said the mare appreciated a return to firmer footing than she has had recently.

"She is such a tough little competitor," she said.

"It all panned out all right. The speed was on which suited her. She got into air when she needed to and tried her heart out, as she does.

"Last-start she went a really good race, but the track was a bit off and she likes it really firm and she got that today. She is just so much happier on good ground."