Racing Victoria has pushed back its timetable for an investigation into elevated levels of cobalt returned in horses from four Victorian stables.

Leading Melbourne trainers Peter Moody, Danny O'Brien and Mark Kavanagh as well as the father-son training partnership of Lee and Shannon Hope are under investigation after horses in their care returned samples with cobalt levels above the allowed threshold.

Racing Victoria's executive general manager of integrity, Dayle Brown, said extensive forensic analysis, analytical testing and interviews have been held over the past two months.

And while RV had hoped to complete the investigations by the end of March, Brown said more time was needed.

"We are working as swiftly as possible with a very well resourced team," he said.

"We have gathered evidence during the course of the four investigations that we wish to further analyse and this will take time.

"It is critically important that we ensure that no stone is left unturned, which must be balanced against the desire of all parties for the investigations to be completed in a timely manner."