Leading trainer Chris Waller has pleaded guilty to a charge of presenting a horse to race with the prohibited substance methylamphetamine its system.

Waller told Racing NSW stewards on Friday he had no idea how Betcha Thinking came into contact with the drug, commonly known as ice, but said he had stepped up internal testing of staff and procedures within the stable.

Betcha Thinking has been stripped of his second placing in a race at Canterbury on October 5 after which he tested positive.