Sightings of creepy clowns that have spooked the United States appear to have sparked 'copy-cat' acts in Australia and New Zealand, with police issuing stern warnings for would-be clowns.

Numerous sightings of clowns appeared on social media in the early hours of Sunday in several Australian cities, with one woman in Brisbane telling local media she was chased by a clown holding a knife.

The clown sightings started around Greenville, South Carolina, in August, when police received reports of clowns standing silently by roadsides, lurking near laundromats and trying to lure children into the woods with bags of cash and green laser lights.

It's unclear what started the craze, although some have suggested it may be part of a horror movie publicity stunt or an elaborate hoax.

Since then videos and pictures have appeared on social media of clowns scaring residents in various U.S cities, as well as in Australia and New Zealand.

In the New Zealand city of Hamilton, a 22-year-old woman was assaulted early on Saturday by two men dressed as clowns, police said.

"This was a very frightening experience for the young woman," New Zealand police said in a statement.

Australian police have issued strong warnings to those dressing as clowns, saying they could be committing criminal acts or become victims if scared citizens attack them.

"The clown purge appears to be a copycat of incidents being seen in the U.S. Any intimidating or threatening, as well as anti-social behaviours will not be tolerated and will be investigated," police in the Australian state of Victoria said in a statement posted on social media.