An inquiry into a positive swab returned by the winner of the feature race on one of Australia's richest days of picnic racing will be held on Monday.

Trainer William Slater will face Racing NSW stewards after Color Purple tested positive to lignocaine after winning the $15,000 Fernhill Picnic Cup in October.

An eight-year-old, Color Purple's victory was his first for Slater and it ended a winless streak of almost four years for the gelding.

The Fernhill Picnic Cup was first run in 2013 when a sprawling property on the western outskirts of Sydney was granted a licence to use a purpose-built racetrack for picnic racing.

Backed by Fernhill Estate's wealthy owners Simon and Brenda Tripp, the meeting is run under the control of the Hawkesbury Race Club.

And despite its short history, the meeting attracts thousands of racegoers as well as owners, trainers and jockeys who are regulars on the NSW picnic racing circuit.

Leading trainer Clarry Conners was hit with a $10,000 fine when a horse he trains for John Singleton tested positive to lignocaine after winning at Canterbury in September.

Lignocaine is described as a local anaesthetic which can have an effect on the nervous and cardiovascular system.