An outbreak of equine influenza has hit Australia's multi-billion dollar horse-racing industry, causing meetings to be cancelled across the country.
Horse-racing is an Australian national pastime
The highly contagious flu, which rarely kills adult horses but can be lethal for foals, was found at stables in the country's largest city, Sydney.
Agriculture Minister Peter McGauran said the thoroughbred industry was facing its biggest-ever risk.
It is the first time equine influenza has appeared in Australia.
The government banned the movement of horses for 72 hours after flu was found in 16 animals at two sites in Sydney.
Only the Northern Territory has been spared the indefinite ban on horse racing, under which severe penalties will be imposed on anyone caught transporting horses.
"There could be no more serious situation, to be frank," Mr McGauran said on Australian TV.
"But we're still optimistic. Nobody is panicking. The industry has not hesitated for a moment to bring in unprecedented measures of a kind that might be financially devastating but will give the containment strategy the best chance of succeeding."
Racing is one of Australia's great national pastimes, the BBC's Nick Bryant reports from Sydney.
Horse-breeding has also been disrupted by the outbreak of a virus which kills about 40% of foals which come into contact with it.
People are immune from the virus.
The suspension of racing will cost millions, not just to the horse industry but to the bookmakers as well, our correspondent notes.
When equine influenza hit South Africa, it led to a five-month cessation of racing and if that happened in Australia it would lead to the cancellation of the Melbourne Cup - the fabled race that stops the nation.
The ban on horse movement could also affect the policing of the Apec meeting next month - a summit of more than 20 international leaders including US President George W Bush.
With major protests planned on the streets of Sydney, mounted police units were due to be drafted in from other parts of the country.