The jockey-less mare was fourth across the line, just when those punters who had backed her were on their way to collect their winnings.
The New Zealander was able to walk unassisted to an ambulance on Wednesday, where he complained of neck pain.
Collett was stood down for the remainder of Wednesday's midweek card.
Prahaar was holding eventual winner Swaffham Bulbeck at bay when she headed to the outside rail.
Inexperienced horses can shift suddenly when the jockey uses the whip, or perhaps the filly saw something to avoid on the track ahead.
Winning jockey Brenton Avdulla took no satisfaction from his Hawkes Racing-trained gelding breaking his maiden at the third attempt.
"He had a good margin on me, he was probably half a length in front of me so it's a bit of an ill feeling for me," he said.
"I'm pretty close to Jason and it's not ideal when you see any rider go (off)."
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