The father and daughter training duo of Terry and Katrina O'Sullivan and three other trainers have been charged with cobalt offences, some dating back more than two years.

The O'Sullivans face nine charges, six in the alternative, after three of their horses returned readings above the 200 micrograms per litre in urine threshold in 2015 and 2016.

Racing Victoria stewards issued charges against the O'Sullivans, Monica Croston, Matthew Leek and Trevor Andrews on Thursday.

Darragh, trained by the O'Sullivans, returned high readings in pre-race samples at Ballarat in November 2015 and again at Sandown in August 2016.

Another O'Sullivan horse Gold A Plenty returned an above-threshold reading at Sandown in August 2016.

The pair face three charges of administration of a prohibited substance for the purpose of affecting the performance or behaviour of a horse in a race.

The six remaining charges in the alternative, relate to the administration of a prohibited substance detected in a race day sample and the presentation of a horse to race with a prohibited substance in its system.

The O'Sullivans were first notified in January 2016 of the confirmed cobalt positive from Darragh's Ballarat sample and in September that year of the other two positives.

Croston faces three charges, two in the alternative, after Hezza Tidal Wave returned a high reading in a pre-race sample before a Sandown race in June 2016.

Matthew Leek faces charges for a high reading from Champagne Kisses before a Pakenham race in March that year and Trevor Andrews over Coppola at Geelong in December 2016.

Like Croston, Leek and Andrews each face three charges, two in the alternative, with all three informed of their high cobalt readings more than 12 months ago.

The charges will be heard by the Racing Appeals and Disciplinary Board on a date to be determined.