A relieved Chris Waller will head to the final day of the Sydney autumn carnival cleared of wrongdoing over positive swabs returned by three of his horses which consumed contaminated feed.

However the company which supplied the feed, Agricure, will be referred to government authorities over its lack of quality control.

Sydney's premier trainer fronted a Racing NSW stewards inquiry on Friday where he pleaded guilty to charges of bringing the horses - Seuss, Arinosa and Jade Marauder - to the races with the prohibited substance ibuprofen in their systems.

The charges place ultimate liability with the trainer but stewards agreed Waller had done everything possible to ensure his horses were drug free.

"It's been a very testing time for the stable," Waller said.

"Obviously our racetrack success has been unaffected but to have this hanging over our heads has not been easy."

Waller had asked his usual supplier if there was a product he could use on horses which had a tendency to tie up and they sourced it from Agricure.

He sought and got assurances the product was all natural and free of any prohibited substance.

The horses were housed in one barn and no positive results were returned from the first batch but the three horses all returned positive results after being fed from the second batch.

The substance was found in high doses in two open buckets of feed and nine sealed buckets taken by stewards for analysis after they were alerted to the results by the racing laboratory.

Dr Ray Biffin, a principal of Agricure, told the inquiry his company had been making an experimental product for a Melbourne vet which contained ibuprofen.

He said it was possible the product containing ibuprofen had gone through the manufacturing mill directly before the feed supplement.

Chief steward Ray Murrihy said his panel was satisfied that Agricure was the source.

"Something at Agricure has gone astray," he told Biffin.

"From what we've heard the quality assurance leaves a lot to be desired.

"There needs to be a rethink if you are using a product with these substances for other clients.

"It could have been a disaster. Already the horses have been disqualified and the ramifications are high for the owners and the trainer."

In recording the conviction without penalty, Murrihy cited precedents including the case of Ortensia who was stripped of a Group One win when contaminated feed was deemed to be the source of ractopamine in her swab.

Her then trainer Tony Noonan was not penalised.