Ms Evans said stock management had been a major challenge this year with feed shortages, lice treatment breakdowns and now blowflies.
"Reports have been received from throughout the northern, eastern and southern agricultural areas with strike occurring very quickly in some instances," Ms Evans said.
"Farmers are experiencing different problems with some reporting sudden deaths from body strike while others are battling to prevent breech strike in dirty sheep where crutching cannot be done.
"One of the biggest challenges is protecting sheep until crutching, whether it is prime lambs destined for market or stock due to be shorn in the near future.
"Farmers are urged to be vigilant and closely monitor stock as some properties have experienced sudden and significant losses."
Ms Evans said applying chemical to dirty breeches was unlikely to offer any real protection as it was likely to be inactivated or unable to compete with the overwhelming attraction of soiling.
"Producers need to monitor and treat individually struck animals until crutching or shearing and if scouring is due to high worm burdens then a drench should be given immediately," she said.
"It is also important to consider the wool withholding period, export slaughter interval and the expected length of protection when selecting a chemical.
"An exemption regarding the wool withholding period applies when treating individually struck sheep."
It is essential to follow label directions and maintain a chemical treatment record which includes batch numbers.
Ms Evans said the main chemicals available for fly control had reasonable export slaughter intervals (ESIs).
Current ESIs can be found on the Australian Pesticide and Veterinary Medicines Authority website www.apvma.gov.au.