The case was among several the Wallaceville-based centre reported on for the period from April to June 2007.
Investigations found no evidence of exotic disease - which is the centre's sphere of responsibility - but the matter is with the Food Safety Authority, it said.
It reported as follows:
"A veterinarian phoned the 0800 number after two yearling colts (a miniature pony and an Arab crossbred) developed respiratory distress and died suddenly. The veterinarian had seen the horses before death, when they were afebrile and in severe respiratory distress.
"The miniature pony developed respiratory signs within 15 minutes of a morning feed, and died two or three hours later. The feed was a mix of chaff and proprietary horse feed, with a small amount of copra (a kiln-dried coconut byproduct) added.
"The crossbred Arab was fed the same ration and developed respiratory signs later in the day. Despite receiving injectable steroids and antihistamines it died in the early evening. Four other horses on the property receiving only supplementary hay remained healthy. The miniature horse was born on the property and the Arab crossbred had arrived three months previously.
"Necropsy of both horses showed similar gross findings: severe hydrothorax, non-collapsing lung tissue, lung oedema and froth in the lower airways. The abdominal contents were normal apart from an enlarged congested liver. Biochemistry and haematology were unremarkable apart from an elevated packed cell volume probably associated with splenic contraction, and hypovolaemia from outpouring of fluid into the lungs.
"Histology revealed severe acute pulmonary oedema, with no evidence of an infectious agent. Liver tissues were moderately to severely congested, and other tissues unremarkable. Histology of cardiac muscle from one horse was normal. The findings were consistent with a direct pneumotoxin, although a cardiac toxin and an anaphylactic (type 1 hypersensitivity) reaction were also considered.
"Clinical, epidemiological and histological findings ruled out the involvement of exotic disease.
"Laboratory testing of the copra and the proprietary horse feed mix for selenium, ionophores and 1080 (sodium monofluoroacetate) by AgriQuality Limited found no evidence of 1080, and selenium levels were within normal limits.
"Two ionophores (monensin and lasalocid) were found in the proprietary feed at levels of 3.7 mg/kg and 0.038 mg/kg, respectively, and none in the copra. These levels are considered insufficient to cause the severe clinical syndrome and sudden death, but the presence of any ionophore in horse feed is considered an indicator of potential contamination.
"Further investigation, including wider testing of the horse feed and feed mill auditing, is under way as part of a New Zealand Food Safety Authority (NZFSA) led investigation."