Owners have a legal responsibility under the Exotic Diseases Act to report any possible cases.
The state's deputy chief veterinary officer with the Department of Primary Industries, Steve Dunn, said reporting ill horses is vital to stop any further spread of equine influenza.
"While most people are doing the right thing and reporting sick horses we are still receiving tip-offs that some horse owners are not reporting," he said.
"Failure to report could have widespread and serious implications in terms of our eradication programme.
"Everyone has a responsibility under the Act to report. Anyone who is found to have ignored this directive faces the full force of the law," Dunn said.
The maximum penalty for failing to report is $A22,000.
Clinical signs of equine flu include a deep dry hacking cough, a watery nasal discharge that may become cloudy or coloured, a sudden increase in temperature, depression, loss of appetite, laboured breathing, muscle pain and muscle stiffness.
Clinical signs must be reported to the NSW DPI hotline on 1800 675 888.