An adult boa constrictor at least three metres long is on the loose outside Warragamba in Sydney's far west.
In a letter circulated to residents of the Cascades Estate in Silverdale, the NSW Department of Primary Industries' invasive species associate director Scott Charlton said a fresh snake skin was located at a Torumba Circuit construction site on October 9.
The letter is dated October 11 but was handed out to residents on Monday.
Mr Charlton said the government believed the skin was that of an adult boa constrictor and was now seeking to catch the snake.
Native to Central America, boa constrictors bite and strangle their prey and are a potentially fatal threat to both animals and humans.
They are banned outside zoos in NSW.
A NSW DPI spokesman on Monday said residents had been encouraged to report any snake sightings.
"Once located, a licensed snake catcher will be contracted to catch the snake and transport it to a specialist veterinarian," the spokesman said in a statement.
"The snake will be examined to determine where it came from, how long it has been in the area, what it has been eating, whether it's carrying any diseases of concern and whether it has produced offspring."
The DPI on its website says boa constrictors come in colour combinations including red, green, yellow and tan - and generally display patterns on its body of jagged lines, ovals, diamonds and circles.
The coloured patterns aid in camouflaging the snake under natural conditions, allowing the species to evade detection and ambush its prey, the DPI says.
They have successfully established themselves as pests in Florida and are considered a "serious" establishment risk in Australia.
© AAP 2019