The brother of controversial jockey Danny Nikolic has been disqualified over his involvement in the raceday treatment of a subsequent country NSW winner.

John Nikolic was disqualified for nine months after he pleaded guilty to administering a banned medication to Wonderful Jester before the galloper part-owned by his wife Yvette won at Murwillumbah in August.

A Racing NSW inquiry held at the Gold Coast racetrack on Thursday and chaired by Ray Murrihy, found that John Nikolic had rubbed an absorbent pad covered in penetrene over Wonderful Jester before the horse was presented in the mounting yard for its Murwillumbah engagement.

Penetrene is a liniment that assists in the relief of muscle soreness and is a prohibited substance in racing.

John Nikolic was also fined $200 for saddling up Wonderful Jester as an unlicensed and unregistered person.

Wonderful Jester's trainer Lisa Garrett and a stablehand were also fined $200 and $3000 respectively for their involvement.

Backed from $7 to $3, Wonderful Jester won the Murwillumbah race by 1-3/4 lengths.

In six previous starts for Gold Coast-based Garrett, Wonderful Jester had been beaten by a combined margin of almost 50 lengths.

Once a member of Nathan Tinkler's Patinack Farm string, Wonderful Jester was sold at auction in 2013 for $90,000 to be trained by John Nikolic.

It didn't take long for the now six-year-old to break through for his new stable, landing a $4.40 to $1.80 betting move at Casino.

The stable pulled off another coup with the horse at Murwillumbah in 2014 only for Wonderful Jester to return a positive swab and John Nikolic was disqualified for 12 months.

Danny Nikolic is trying to save his career in the saddle after a history of run-ins with Victorian racing officials.