Two men poured petrol on a two-year-old boy and threatened to set him alight in front of his mother during a robbery in Launceston, in Tasmania's north, police say.

Key points: Police describe home invasion as 'horrific'

Police describe home invasion as 'horrific' Two men in balaclavas doused mother and two-year-old son in petrol

Two men in balaclavas doused mother and two-year-old son in petrol The pair wanted to know location of two other men who weren't at the house

Petrol was also poured over the boy's 23-year-old mother by the men, who were wearing balaclavas when they forced their way into the woman's home on Kay Street, King Meadows, about 7:00am on Wednesday, police said.

Investigators described the incident as "horrific".

They believe the men were looking for two people and when they were not there, they threatened to harm the mother and son and burn down the house.

They eventually left the premises with a sum of cash.

Acting Detective Inspector John Parker said the men came through the backyard and forced their way through the front door.

He said the men threatened the victim with assault and took her into another room.

The toddler was put in a cot by two women who were at home at the time.

"When the offenders have not been able to ascertain the information they wanted, they've poured petrol over the victim and then they've poured petrol over the back of the two-year-old child and threatened to burn the child in the presence of its mother," he said.

"Whether it be the mother or the child … the fact that someone has had petrol poured over them and then threatened to be burned alive is a horrific act."

Police are calling for witnesses and anyone with CCTV footage in the area.

The toddler was not injured, but the woman was taken to hospital for assessment.

"The child is fine, the petrol was only over the clothing," he said.

"The mother ... appeared fine but as the day has gone on she's has had some complications from the petrol."

Police would not comment on whether the incident was drug-related and said the offenders were known to the occupants.

"They were trying to locate these other two persons who they believed to be there and were threatening her with her life," Acting Detective Inspector Parker said.