A Chris Waller-trained horse has reportedly tested positive to methamphetamine.
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The Sydney trainer will await the results of a second test before potentially facing a stewards' inquiry.
Waller, trainer of superstar Winx, said the stable were in the process of investigating the situation.
"We are working through the possibility of a positive sample to ice from one of our horses," Waller said.
"It is an issue we want to get on the front foot with and we have tested our staff to try and find how this happened.
"We want to work out where the contamination has come from and whether it was from one of our staff or an outside influence."
A trainer in Victoria avoided suspension last year after a horse tested positive for ice when it was found a stablehand had been using the drug.
Waller has previously had doping cases brought before Racing NSW.
In 2013 he had three positive tests for ibuprofen and in 2014 he was fined $30,000 after Junoob popped for the diuretic Frusemide, also known as Lasix.