The state's Department of Primary Industries said that while there were two positive tests just before Christmas, antibody levels in both horses placed the infection date early in December.
With no active infection in many of the red zones for some time, authorities are hoping to improve the status of these zones, and their surrounding buffers, in the near future.
The conditions allowing a red zone to improve to amber status are:
"A lot of this in now in place for most of the 'isolated' red zones in NSW, with the exception being the Grenfell area," a spokesman said.
"Assuming that there are no fresh infections, and surveillance test results are as expected, the red zones around Parkes, Forbes, Dubbo, Wellington, Narrabri, Milli, Armidale and Walcha could be rezoned as amber before the end of January.
"Again, assuming everything goes to plan, these zones would be rezoned Green a month later."
Eradications efforts in Queensland are also encouraging, with a further easing of horse movement restrictions in south east Queensland.
Some repeated horse movements by float or truck and 'on hoof' movements within local areas are now allowed in the red zone under special conditions.
New 'On Hoof' and 'Multi-Movement by Vehicle Permits' are designed to help horse owners and businesses get back to normal operations.
Department of Primary Industries and Fisheries chief inspector of stock Dr Allison Crook said horse owners could now walk horses off a property without a permit as long as several conditions were met.
"However, on-hoof movement of horses can only be done during daylight hours and they must be returned to the property of origin the same day," she said.
"Horses cannot be moved if they are sick, and must not be taken from or to, an infected or suspect property.
"The on-hoof movement will allow local movement of horses where, for example, an owner wants to ride to a local showgrounds or pony club grounds to exercise or train a horse."
Dr Crook said overnight or weekend stays at a destination premises were not allowed and such movements still required a permit.
Horses don't have to be vaccinated, microchipped or blood-tested for On-Hoof movements.
Two new permit types have also been launched to help horse owners with regular normal movements between specified destinations and the home property, the details of which are available at www.dpi.qld.gov.au.
Chief veterinary officer Dr Ron Glanville said the easing of movement restrictions was an acknowledgement of progress that has been achieved with the disease eradication programme. "This has been due to the horse community's compliance with movement restrictions and good biosecurity measures since the start of the EI outbreak in August last year," he said.
"We know that this easing of movement restrictions may reveal some hidden pockets of infection in some areas.
"We have containment and suppression vaccination plans in place depending on the severity and location of any new outbreak that may result."
The department can also suspend or revoke all permits until further notice to allow vets to assess and manage any new flair up of the virus, he said.
It was essential that owners respond quickly if they detect signs of respiratory illness in their horses by calling the department, he said.
"Good biosecurity and decontamination are still essential. Horse owners must not move sick horses, or allow them to come in contact with other horses."