Australian authorities have launched their battle plan in a massive effort to contain an outbreak of highly contagious equine influenza.
A 72-hour nationwide ban on horse movements was to begin at 1pm on Sunday.
Sixteen horses have tested positive for the virus so far, with further cases suspected. Test results are awaited.
The outbreak has thrown Australia's racing and sport horse industry into turmoil.
The crisis began when two quarantine facilities - one west of Sydney at Eastern Creek and the other at Spotswood, Victoria, were locked down after a stallion tested positive for the virus at Eastern Creek. The second facility was locked down because some of the horses had travelled to Australia together.
However, soon after the disease appeared at Sydney's Centennial Park stables, where at least 11 horses have tested positive to the virus. Authorities have created a 10km exclusion zone around the stables.
The zone includes Randwick racecourse. The area is thought to be home to at least 700 horses.
From 1pm Sunday, all horse movements across Australia have been banned for 72 hours, pending an assessment of the ongoing risk.
In weekend developments:
Australia and New Zealand remain the only two countries with major equine populations that have remained free of the disease. Both nations have an equine population that has not been vaccinated and will have no resistance to the disease.
Its spread would see many thousands of horses contract the disease. While the respiratory disease is not fatal, veterinarians says there is a higher incidence of serious complications with equine influenza, compared with other respiratory illnesses. Complications include heart problems and pneumonia. Horses can take many weeks to regain condition after a bout.
Federal Agriculture Minister Peter McGauran told media an iron curtain has fallen. "There could be no more serious situation, to be frank," McGauran said.
"We've never had an introduction of influenza in the equine population of Australia ever before. If it was to escape beyond the stable of the horses in Centennial Park it would have a devastating effect on the horse population."
Race meetings across New South Wales have been cancelled in the wake of the outbreak, in a move which will cost the industry millions of dollars. Meanwhile, racing will go ahead in Victoria.
The outbreak is a double blow to the racing industry, with more than 25 shuttle stallions under lockdown in two quarantine facilities after one of them tested positive to the virus after arriving in the country.
It is uncertain what ramifications the outbreak will have for this year's running of the Melbourne Cup, but the lockdown would have to remain in place for only a few weeks to begin affecting the preparation of the runners. Questions also hang over the Japanese defence of the Cup, with last year's winner Delta Blues and runner-up Pop Rock in quarantine in Japan, a country also working to contain an outbreak.
Agriculture Minister McGauran said: "We won't allow ourselves to even consider the worst case scenario because we are working to contain it - otherwise the worst case scenario will eventuate."
The ongoing quarantining of the international stallions has thrown the breeding plans of at least 2000 of Australia's top thoroughbred broodmares into disarray in the short term.
The connections of the horses can either seek services to alternative stallions, or wait a month for the stallions to be released, this being contingent on no more signs of the disease at the quarantine facilities in question. However, the stallions may not be able to meet the additional demands of a backlog in time for this breeding season.
There are plans for the stallions to service hundreds of mares, with service fees in some cases approaching $A200,000.
Australia's Consultative Committee on Emergency Animal Diseases (CCEAD) announced the nationwide ban on horse movements as part of a coordinated national response consistent with Australia's veterinary emergency plan.
"The purpose is to reduce the risk of spreading infection by avoiding congregations of horses from different origins. Racing clubs and other equine associations are being asked to cancel or postpone any meetings or events.
"Owners are also being asked to limit contact between their horses and other horses as much as possible and not to move their horses.
"For events already under way, organisers are requested to contact the State Department of Agriculture regarding advice on how to manage the standstill in relation to these events. For states other than NSW, some movement of horses in transit will be allowed to continue, if they had already left their usual place of residence. They will be able to return provided that the journey can be completed expeditiously."
However, in NSW, no movements of horses are permitted at all.
MAF Biosecurity New Zealand Assistant Director General, Dr Barry O'Neil said New Zealand has moved quickly and adopted a cautionary approach, to what is an evolving situation.
"New Zealand is one of the few countries in the world that is free from Equine Influenza. MAF Biosecurity New Zealand has moved swiftly to put measures in place to protect our equine industry from this disease. We are working closely with the Australian authorities, the New Zealand equine industry and our veterinary association, and will review our position over the next two or three days, as more information comes to light."
All horses that have arrived in New Zealand since the beginning of August will be identified and their movements traced. MAF Biosecurity New Zealand has a comprehensive response plan in place, should the disease ever be found here.