With its striking black mohawk and red cheeks, the red-whiskered bulbul could be mistaken for a bird-watchers' delight.

But instead, this bird has become one of South Australia's "most wanted", prompting concerns for the state's $2 billion horticulture sector and highlighting what the Invasive Species Council calls a gap in the nation's biosecurity.

Authorities have been searching for the single red-whiskered bulbul since it was spotted in Adelaide's north several weeks ago — having likely escaped captivity despite being illegal to keep in the state.

The invasive species has become established in parts of NSW and Queensland, where it is damaging crops, spreading weeds and competing with native birds.

Red-whiskered bulbuls are native to southern Asia, but introduced to Australia in the early 1900s. (Supplied: Jay Yeung.)

Invasive Species Council senior advocate James Johnson was concerned there could be more out there.

"This could be a start of a new invasion if we don't act quickly," Mr Johnson said.

"That species of bird is banned here in SA, so it's been likely brought in by someone wanting to keep it as a pet.

"We haven't found this bird yet, so we're actually calling on the public to keep an eye out and report any sightings."

Mr Johnson said the bulbul was implicated in the spread of weeds like feral olive trees in Western Sydney, and posed a major risk to the environment and agricultural areas.

Invasive Species Council SA-based senior advocate James Johnson. (Supplied: Invasive Species Council.)

"It's been spotted in Salisbury — that's not too far from our agricultural areas in the north of Adelaide and it could very easily get into these fruit picking areas and do a lot of damage," he said.

Authorities hunting bird

In a statement, a spokesperson from South Australia's Department of Primary Industries (PIRSA) said the SA sighting was first reported in late November.

"The red-whiskered bulbul is a high-risk pest bird that can damage SA's vineyards and orchards by eating soft fruit, flower buds and insects, potentially reducing yields or causing crop failure and threatening the state's $1.8 billion wine and $2.08 billion horticulture industries," the spokesperson said.

"Established populations have caused significant problems for soft-fruit and citrus growers interstate and overseas, highlighting the risk and cost if they become established here."