While TAB bookies will not disclose the punter's name or whether it is the same person, there have been a series of huge bets on hot favourites, with Dunn the most regularly supported, in the Driver's Challenge this year, more regularly over the winter.
With the pool of gamblers in New Zealand who would place a $10,000 racing bet being very small, it seems logical it has been the same punter cashing in on Dunn.
The Canterbury horseman is often successfully smashed for a five-figure sum at around $1.30 or $1.40 at Forbury Park meetings and he admits he knew about the $15,000 bet halfway through Sunday's meeting.
"I heard Greg O'Connor mention it on Trackside but, obviously, it doesn't affect anything I do," says Dunn.
"When you hear something like that you are hoping you can win for the punter involved because that is a lot of money but then again, I am always hoping I can win."
Dunn said two weeks ago, when he was on 178 winners for the season, he hoped to reach the 200 mark, something he has missed by one win the past two years.
Now, even the uber modest 24-year-old says the double century looks a certainty. "I've got 11 meetings here in the South Island before the end of the season and only need eight more wins.
"So the way I am driving and the sort of horses I am getting on, unless something goes wrong I would be disappointed not to get there."
Dunn's form isn't putting smiles on the face of bookie Steve Richardson, who is copping a double whammy. "Dexter tends to be very heavily backed in the Driver's Challenge, no matter how short we open him.
"And then, of course, we take money for the favourites he is on in the race-by-race markets."
Richardson was left stunned by two strong weeks of favourites recently, which saw more than 60 per cent of them win in New Zealand harness races two weeks in a row.
"Over the course of the season is it usually about 36 per cent of the favourites win but we had a horror fortnight," said Richardson.
"With those sort of percentages winning and the way the gallops tracks are, I can see why we are getting hit hard."
While Dunn is eying up his third 200-win season, he is more excited about the shape his star drive Christen Me has returned in for trainer Cran Dalgety.
"He is definitely bigger and stronger than last year," he says of the Hunter Cup winner.
"He is hoppling along slowly now but we couldn't be happier with him."
Meanwhile, Dunn's father Robert needs six more training victories for the season to reach the 100 wins in a season for the first time, having already surpassed his previous best of 90.
