Cliff Mapham dropped his son Phoenix off at Orana Steiner School in Weston last Thursday and hasn't seen him since.

Police believe Phoenix's mother, Tessa Woodcock, who does not have custody of the boy, unlawfully took him from Hickey Court in Weston, about 1.30pm last Thursday.

While police have refused to discuss the circumstances around Phoenix's disappearance, Mr Mapham said the six-year-old was taken from the school oval in front of other children.

"I just want to know if he's safe, that's my main priority," Mr Mapham said, urging anyone with information to contact police.

Mr Mapham said he had the day off work last Thursday and had planned to buy his son a fancy dress costume for a school event.

Nearly a week after Phoenix disappeared, there has been no sign of him or Ms Woodcock, leaving Mr Mapham at his wit's end and desperate for answers.

Mr Mapham, who said he had been granted full custody of Phoenix about two months ago, described his son as "a very caring boy who would go out of his way to help if someone was upset".

"He's a very active boy and he's always trying to be funny and make people laugh," Mr Mapham said.

"He's not backwards in coming forwards and he loves talking to everyone, even strangers.

"He likes his cooking, tuning his ukulele and he loves knitting, drawing pictures and his go-kart."

Police searching for Phoenix and Ms Woodcock have issued the ACT's first ever amber alert - an urgent nationwide broadcasting system used to help find a child in an abduction or high-risk missing child case.

Investigators have said they believe the pair to be either in the ACT or in NSW - possibly in the South Coast region.

On Wednesday afternoon, Detective Station Sergeant Harry Hains appealed directly to Ms Woodcock, saying police had serious concerns for Phoenix's safety and wellbeing.

"Tessa, we need to know that your son is OK," Detective Station Sergeant Hains said.

"We need to see him. We are talking about his health, we are talking about his safety.

"You need to bring him to a place where we can verify that you are taking care of him.

"That could be a hospital, a doctors’ surgery, a health facility or a police station. Please bring him in."

Detective Station Sergeant Hains said police were actively searching for mother and son, but would not detail any areas police were focusing on, or any leads they were following.

He said it was possible that Ms Woodcock's associates knew where the pair were, and urged them to contact police if that was the case.

"The fact of the matter is that we have a six-year-old boy who has been taken by his mother," he said.

"That boy has now not been seen for six days.

"That, in itself, raises serious concerns."

Police say Phoenix was last seen in a white 1999 Holden Vectra sedan, believed to have been driven by Ms Woodcock, with registration YAU56P.

They may also be travelling in a blue Subaru Forester or similar vehicle with a broken rear window, damaged front end and possible registration YKP08U.

Detective Station Sergeant Hains urged anyone with information to call police on (02) 6256 7777.

Information can also be provided anonymously to Crime Stoppers on 1800 333 000, or via the Crime Stoppers ACT website.