Fears for missing Canberra boy, 6

A mother who is believed to have unlawfully snatched her six-year-old son in Canberra six days ago is being urged to contact authorities.

"We need to know that your son is OK. We need to see him," Detective Station Sergeant Harry Hains told reporters in Canberra on Wednesday.

Police are seriously concerned about the wellbeing of Pheonix Mapham, who was last seen about 1:30pm on Thursday in a car with a woman believed to be his mother, Tessa Woodcock.

The 1999 Holden Vectra sedan was being driven by a woman in the Canberra suburb of Weston Creek.

The pair are thought to be in either the ACT or NSW, but authorities have issued a national alert appealing to the public to help find the boy.

It's the first time ACT police have issued such an alert and they say Ms Woodcock should not have custody of Phoenix.

The boy and his mother could also be travelling in a blue Subaru Forester with a broken rear window.

Sergeant Hains said he hoped Ms Woodcock would take her son to a hospital, doctor's clinic or police station to verify she is taking care of him.

"We are talking about his health. We are talking about his safety," he said.

The detective could not comment on the child's custody arrangements but stressed he was taken unlawfully.

"I urge you Tessa, I urge you to get in contact with us and bring Pheonix in."