A man and a woman are in a critical condition in hospital after suffering suspected carbon monoxide poisoning while burning charcoal inside their home as temperatures plummeted in Perth.

Emergency services have taped off a house in Westminster, in Perth's north.

Department of Fire and Emergency Services district-officer Luke Miles said it was believed the couple, thought to be in their 50s or 60s, were using a charcoal burner.

"The information that we currently have based on what we've located and seen was the burning of charcoal in an unventilated area," he said.

"So for a long period of time, they've been exposed to potential carbon monoxide poisoning."

Perth's temperature dipped to a low of 3.4 degrees Celsius overnight, about 5 degrees lower than the June average minimum of 8.6 C.

DFES was called to the residence at about 12:45pm, after St John Ambulance personnel went to the scene and found two people unconscious.

Mr Miles said crews responded to what was believed to be a hazardous situation.

"Crews arrived, they provided atmospheric monitoring, ventilation and support to St John to make the area safe," he said.

Charcoal burner in bedroom

Mr Miles said it appeared the couple were trying to keep warm overnight.

“They were using it as a heating source in a bedroom, so there was actually a double bed where both of the casualties were located,” he said.

Mr Miles said children were present in the home but were not injured.

He said it was an uncommon occurrence for people to burn charcoal as a heating source.

"The charcoal was in what you'd normally find in one of those sort of incense burners on a larger scale," he said.

"Obviously, the safe use of it is a common practice in a well-ventilated area. It's not often that we get a lot of these incidents, but it's not unheard of."

He said the family had recently moved into the property and had been up late unpacking boxes.