"I can't believe this has happened to you, one minute we were having a laugh in the gates together, the next we were both in trouble doing what we both love," she wrote.
"I'll miss seeing your bubbly happy smile at the races ... Getting back in the saddle today and gonna try ride a winner or two for you Ashlee Marie Mundy."
Hours before Miss Direen raced in honour of her friend, Miss Mundy's family were gathered at her bedside at Dunedin Hospital.
After the accident she was put on life support but even after her family made the decision to turn the machines off they remained on until her organs could be harvested for donation.
Miss Mundy's father Lockey Mundy posted a photograph on his Facebook page, showing his daughter hooked up to life-sustaining equipment. Her mother Cheryl, partner Brad Frew who flew to Dunedin after hearing about the accident, and other family were with her.
"We are still here at the hospital with her as we await the arrival of an organ donor team so in death she can give hope to others," Mr Mundy wrote.
It is understood Miss Mundy's body was being taken home to Westport yesterday. Her mother attended the race at Waikouaiti, where a minute's silence was held in honour of the fallen jockey.
Mr Frew told the Gold Coast Bulletin his girlfriend would be "sadly missed".
Miss Mundy grew up in Westport and attended Buller High School. She had been living on the Gold Coast but had returned home for Christmas - her first in about 10 years with her family, including sister Mya, 12, and brother Kane, 9.
She had been excited about staying in New Zealand for a month-long stint riding on the South Island's summer circuit.
Miss Mundy started her jockey apprenticeship in March 2004 for Christchurch's Michael Pitman.
She won her first race in August that year and went on to ride 232 winners in New Zealand and 43 in Queensland.
Miss Mundy's former school teacher Susan Tansey wrote yesterday: "Ashlee Mundy was such a lovely girl. I taught her 20 years ago as a 5-year-old in Westport and she made such an impact with her kindness even at that age that I still remember her as one of the nicest kids I have taught in my career. So sad."
A funeral service will be held for Miss Mundy on Friday in Westport.