Equestrian Sports New Zealand (ESNZ) told the sport's world governing body that it was strongly opposed to the re-introduction of non-steroidal anti-inflammatory drugs (NSAIDs) in the sport.
It said New Zealand would continue with a zero-tolerance policy towards such drugs, and indicated the country may reconsider sending a team to the 2010 World Games if the progressive list remained in place.
ESNZ chief executive and secretary general Jim Ellis laid out New Zealand's position in an open letter to FEI head Alex McLin. The letter was copied to FEI president Princess Haya.
"ESNZ voted against the progressive list in Copenhagen and continues to strongly object to the re-introduction of NSAIDs in our sport," Ellis wrote.
"New Zealand has a fundamental and unequivocal belief that the use of NSAIDs in competition is detrimental to the standards of horse welfare that we set in this country," he said.
"Their use also compromises the image of our sport with competitors, supporters, sponsors and public alike."
World equestrian sport has been embroiled in controversy since the FEI's General Assembly voted in Copenhagen by a narrow margin last month to permit low levels of certain anti-inflammatory drugs, including phenylbutazone, in competition horses.
The list, which was to be introduced on January 1, effectively brought to an end the zero-tolerance policy towards such drugs which had been in place for about 20 years. However, following concerns raised by leading equestrian nations, the FEI has decided to delay its introduction until April 5, pending further consultation.
Ellis said the ESNZ board, which met on Wednesday, felt the process by which the progressive list was introduced into the agenda and papers of the General Assembly at such a late stage was unacceptable to the New Zealand body.
"Our international federation should be working to the highest standards of governance and the late notice of this list, which did not allow for adequate consultation time with our veterinarians and members, has done the FEI a significant disservice," he said.
"Our understanding is that the progressive list was added to the agenda without the formal support of either the FEI Bureau or the FEI Veterinary Committee. This demonstrates a further lack of good process in the presentation of important matters to the General Assembly.
Ellis said the subsequent controversy surrounding the progressive list has had a damaging effect on the introduction of the FEI's Clean Sport initiative, which is widely supported by national federations, including New Zealand.
"ESNZ will continue to maintain a zero tolerance for these NSAIDs within our national medication control programme," Ellis said.
Ellis said New Zealand supported the decision to postpone the new list's introduction to allow "a fuller debate" at the 2010 General Assembly in Chinese Taipei.
"New Zealand supports this postponement as it ensures that NSAIDs will not be re-introduced to the sport during 2010 and hopefully not beyond.
"The proposed postponement would ensure a 'clean' World Equestrian Games in Kentucky.
"In the event that a majority of national federations do not support the postponement, the board of ESNZ reserves its right to consider the participation of the New Zealand Team at WEG 2010."
Ellis said ESNZ's stance reflected the strength of feeling of the equestrian and wider horse sport community within New Zealand.
"Our support for a zero tolerance approach to NSAIDs, is as you are already aware, shared by a large number of national federations around the world; ESNZ hopes that these views are reflected in the future policy of the FEI."