As well as Tessa, Phoebe Howson (Napier) and Aorangi Ragtime, Lily Moore (Poukawa) on former Pony of the Year Fun House and Shanae Savannah (Silverdale) on Tiger Tale LS jumped flawlessly to remain on four faults.
Places second to fifth were all on four faults, so rankings were decided by time.
Phoebe was second in 48.64 seconds, Lily third in 54.07, Keira fourth in 54.42 and Shanae fifth in 59.8.
Annie Moffett (Napier) and Piripiri were sixth on eight faults.
Tessa inherited Mr Acho three years ago from sister Ruby, who enjoyed plenty of success aboard him.
“I knew I just had to ride the course and trust him. I knew he would jump it,” said the Sally Clark-trained youngster. “He could jump the moon, you know.”
The win was special for Tessa.
“I feel like I am following my sister Ruby's footsteps but she was 13 when she first started jumping grand prix, and I am only 11, so it is a big achievement for me.”
Tessa feels like she has been jumping “forever” and loves nothing more than to be on her pony sailing over obstacles.
“He's really cool,” she says of Mr Acho. “Although he can be an angry pony, he has a great personality. I love him.”
Tessa and Mr Acho were late entries, deciding to compete in the grand prix to give them more starts before the Horse of the Year Show in March.
More on the champs, pages 30-31