A horse trained by Terry and Karina O'Sullivan has returned an illegal level of cobalt for the second time.

Darragh, who is already the subject of a Racing Victoria investigation for his high level recorded in November last year, is one of two horses from the stable to return elevated levels in August.

A pre-race sample taken from Darragh before he finished second last at Sandown on August 24 showed a level of 960mcg per litre of urine, almost five times the legal limit.

Gold A Plenty's sample taken before the Crisp Steeplechase on August 7 returned a level of 1200mcg/L.

Gold A Plenty fell and did not finish the race.

From September 1, the cobalt threshold has been reduced to 100mcg/L.

The father-daughter training partnership has been told of the findings and the investigation is ongoing.

An inquiry into Darragh's reading of 1300mcg/L from November last year has yet to be held.