Three horses, trained by a prominent former jockey and an equestrian in Waikato, are the subject of an investigation after one tested positive for excessive levels of cobalt.
Cobalt is a naturally occurring substance but excessive levels stimulate red blood cell production, which increases a horse's speed and stamina.
The Racing Integrity Unit has compared the performance-enhancing effect of excessive cobalt to that of the drug EPO, which has been controversially used by some cyclists.
The horses - Quintastics, Suffire, and Sound Proposition - were trained by former jockey Lance O'Sullivan and equestrian Andrew Scott from Wexford Stables in Matamata.
Quintastics tested positive for excessive cobalt after winning at Matamata on 11 March.
In a Facebook post for Wexford Stables in Matamata, the trainers said the test results were a "mystery" and "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."
The Rugby Integrity Unit said it was awaiting the results of urine samples for Suffire and Sound Proposition, which had been sent for analysis.
General manager Mike Godber said the investigation could take months to complete, after which a decision on whether to lay charges would be made.
"Excessive levels [of cobalt] are considered performance enhancing because it stimulates red blood cell production, very much like EPO and therefore it increases the stamina and speed of the horse, which is in breach of the rules of racing."
Mr Godber said a judiciary panel would then make a decision and it could issue a monetary fine or disqualify the trainers from the industry.
New Zealand Thoroughbred Racing chief executive Greg Purcell said it conducted about 3900 tests on horses annually.
"This is the first positive swab detected since we've been testing for cobalt since last year."
He said the industry spent about $9 million on integrity services a year including stewards, judicial control authority and laboratory for drug testing.
"We've had a total of five positive swabs over the past four years," he said.
"We're confident the use of cobalt or any other prohibited substance is minimal."