Police arrived at an Elm Street address in the early hours of Tuesday morning, but no one answered when they knocked on the front door.

Lake Illawarra Police District Sergeant Peter Northey said a light in the garage then piqued the officers' interest.

"Police have seen ... there was a light on in the garage down the side of the house," he said.

"Police have gone down to the garage to check on the welfare of anybody inside. As they've entered the garage they've found a number of cannabis plants."

Police will allege they found 15 plants growing in a hydroponic set-up. After speaking to the home's occupants, police learned the 12.10am emergency call came from a five-year-old girl.

The Mercury understands the girl asked for police and sounded distressed, before the call was terminated.

"Whatever was said on the phone call was enough to alarm the triple-0 operator, who decided to send police anyway to check on the welfare of the residents," Sgt Northey said.

"With a call such as this, the triple-0 operators are trained to listen out to the background to try and [determine]: 'is this someone needing help and they just can't talk?', or is it just a prank.

"We've since found they were just making a prank telephone call."

The girl's mother, aged 30, is now assisting police with their investigation. The Mercury understands the woman is known to police for minor, unrelated matters.

Police are expected to lay a charge of cultivating a small quantity of a prohibited plant.

Sgt Northey said the incident was highly unusual.

"In my 18 years of policing, this is the first time that this sort of circumstance has happened, but we're really grateful that it did happen," he said.

"Drug possession and cultivation is a serious crime. We do encourage any members of the public that may have information of other homes involved in this activity to notify police by phoning Crime Stoppers on 1800 333 000."