Another Danny O'Brien-trained horse, Bullpit, has returned a high level of cobalt.

A swab taken from Bullpit following his win at Moonee Valley on December 19 showed a level higher than the allowed 200 micrograms per litre of urine.

O'Brien was told in January that three of his horses - Bondeiger, Caravan Rolls and De Little Engine - had tested positive to cobalt.

The threshold was brought into Victorian racing last April with the Australian Racing Board making it national from January 1.

O'Brien's fellow Victorian trainers Peter Moody, Mark Kavanagh and the father and son partnership of Lee and Shannon Hope are also under investigation for cobalt breaches.

Racing Victoria stewards had initially hoped to have their investigation wrapped up by the end of March but have now extended that time frame.

Newcastle trainer Darren Smith has been found guilty of more than 40 charges relating to the administration of cobalt and is awaiting penalty while Sam Kavanagh faces a NSW stewards inquiry following the detection of cobalt in Midsummer Sun after his win in the Gosford Cup in January.

Two as yet unnamed Queensland trainers have also had horses return illegal levels of cobalt.

Cobalt is an essential mineral but when supplemented the substance can alter a horse's blood pattern and can be toxic in high doses.

O'Brien tweeted that Bullpit had received similar supplements to his other horses which have breached the threshold and said he would continue to co-operate with the RV investigation.