A horse from Lance O'Sullivan's stable has tested positive for cobalt, the first case of its kind in New Zealand.

The NZ Racing Integrity Unit confirmed on Tuesday it had begun an investigation into a positive sample returned by Quintastic after she won at Matamata on March 11.

Two other horses from Wexford Stables, which O'Sullivan runs with his training partner Andrew Scott, have also returned irregularities with the samples sent to Perth for further analysis.

Quintastics's sample was above the allowable threshold of 200mcg per litre of urine while the other two swabs in question were taken from Suffire after she won at Tauranga on February 5 and Sound Proposition after his third in the New Zealand Derby.

Positive tests to cobalt have surfaced in Australia over the past year with several high profile trainers either banned or facing charges.

NSW trainer Darren Smith has been disqualified for 15 years, North Queensland trainer Glenn Baker for two years and WA's Sharon Taylor for 18 months.

Sydney trainer Sam Kavanagh and Melbourne trainers Mark Kavanagh, Danny O'Brien and Lee and Shannon Hope have been charged with cobalt offences along with vet Tom Brennan.

A post on Wexford's Facebook page said investigations would continue into the feeding and supplement regime at the stable.

"The matter of how these horses have provided irregular samples is a mystery to us and is highly upsetting to everyone involved. We will now be conducting a thorough investigation into our feeding and supplement regime but as this is an ongoing investigation we can offer no further comment at this stage," Wexford said.

Former champion jockey O'Sullivan is a member of one of New Zealand's most prominent racing families with his father Dave a champion trainer and his brother Paul a successful trainer in Hong Kong.