Equestrian Sports New Zealand says it has received notification the second sample taken from the horse ridden by Jock Paget has again tested positive for a banned substance.
The horse, Clifton Promise, gave two blood samples during racing at the Burghley International House Trials and was ridden to victory by Paget in early September.
The first test was positive last month and on Wednesday it was revealed the second had also shown the presence of the banned substance, reserpine.
Equestrian Sports New Zealand's chief executive Jim Ellis said Paget is confident of providing a comprehensive explanation to the International Equestrian Federation (FEI).
"Jock has to provide evidence to the FEI ... explaining not only how the banned substance has come to be present in the horse's bloodstream, but also explaining that there is no negligence involved or anything like that," he said.
Paget has been stood down from any official Equestrian Sports New Zealand activity while the legal process is underway.
