The win surprised everyone except those most in the know the filly had looked quite backable after a solid fourth in a lower grade race on Wednesday.
As well as the milestone of winning the 150th race, Spring Cheer's win means Wanganui has tasted success in four of the past six Group Three NZ Cups.
In 2008 Hoorang was the winner, owned and trained by the retired Ian Shaw, with the added prestige of the cup being presented by the visiting Princess Anne.
Hoorang was ridden that day by Kelly Myers, Rosie's sister. The 2011-2012 Cups saw stunning back-to-back success for Blood Brotha on both wet and dry tracks the local horse trained by Whangaehu's Raymond Connors and co-owned with his father Mark.
In fact, it was an interaction between Connors and Myers during the Wanganui Jockey Club awards on September 15 which first publicly revealed Myers' quiet confidence in Spring Cheer.
Connors had accepted Blood Brotha's trophy for Stayer of the Year and suggested the gelding would like a crack at a Cup three-peat.
"Good luck with that Raymond, you only have to beat Spring Cheer," said Myers when he took the stage to accept the Trainer of the Year prize.
Blood Brotha was later canned from being a Cup starter.
Even the Oaks Stud crew had their doubts Myers had found the magic formula with Spring Cheer, but in the euphoria of victory the stud's manager Rick Williams described the trainer as "nothing short of a genius".
"When he got this filly to train we didn't hear from him for months and then I got a call telling me to back her in a race at Blenheim," Williams said to media on Saturday.
"Here we are today and he's damn well done it. He might talk in riddles but when he sets a horse he's never far wrong."
The victory makes Spring Cheer one of the youngest-ever Cup winners - and a historic family double for the Myers sisters as winning jockeys.
It was an excellent week overall in the Canterbury sun for Myers-trained horses.
In total, he had five winners over the three-day carnival, which included Prestigiosa taking out the final race on Saturday.
On Wednesday, the Myers-owned Scapolo claimed the $230,000 Couplands Mile with Scapolo having been purchased from an Oaks Stud sire at the Karaka yearling sales.