Jockey David Walker is facing a second charge of betting on another horse in a race he rode in.

On Wednesday it was alleged Walker, 38, placed a head-to-head bet on Sam I Am at Waverley on July 31, in a race where he rode Lil Mer.

Sam I Am subsequently finished seventh while Lil Mer was 10th.

Last week Walker was charged with placing a head-to-head bet on St Ransom in a race at Awapuni on August 16 in a race in which he was riding Watch Your Man.

The Racing Integrity Unit contends Walker placed bets on St Ransom and deliberately rode Watch Your Man to ensure his bets were successful.

Under New Zealand's racing rules, a rider may only bet on the horse he is riding.

Walker is understood to have admitted placing both bets but will defend himself on charges he failed to ride his own mounts out, NZ Racing Desk reported.

A hearing has been scheduled for September 18 at Trentham racecourse in Upper Hutt.

Walker has had a successful career as a jockey.

Among his victories were Group One triumphs aboard C'est La Guerre in the 2008 New Zealand Derby, Legs in the 2006 New Zealand Oaks and Porotene Gem in the Levin Classic.

His father Jim Walker was best known for riding former New Zealand and Australian Horse of the Year Veandercross.