The father of a six-year-old Canberra boy who has been missing for almost a week has pleaded with the public to help him bring his son home.

Key points: Police believe Phoenix Mapham was last seen on Thursday in a car with his mother, Tessa Woodcock

Police believe Phoenix Mapham was last seen on Thursday in a car with his mother, Tessa Woodcock Ms Woodcock did not have custody of her son

Ms Woodcock did not have custody of her son Police said Phoenix was taken in a white Holden sedan, but could also be in a blue Subaru SUV

Police said they have serious concerns for the safety of Phoenix Mapham, who was last seen about 1:30pm on Thursday in a car with a woman believed to be his mother, Tessa Woodcock.

In a statement, ACT Policing said Ms Woodcock should not have custody of Phoenix.

Police believe Phoenix is with his mother, Tessa Woodcock. ( Supplied: ACT Policing )

The pair were seen driving along Hickey Court in the south Canberra district of Weston Creek, and police believe them to either still be in Canberra or in New South Wales — possibly on the south coast.

Concerns escalated on Tuesday afternoon, prompting police to issue the territory's first amber alert, reserved for high-risk missing person cases where time is vital, or in a child abduction case.

Phoenix's father, Cliff Mapham, said he was buying a dress-up costume for his son to wear for his school's book week event when police called with the news of his disappearance.

"I took him to school on Thursday morning, told him I'd be there after school to pick him up as I had the day off [work]," Mr Mapham said.

Canberra man Cliff Mapham says he feels helpless over his missing six-year-old son, Phoenix. ( Facebook: Cliff Mapham )

"My partner and I — we ran out that store, trying to do everything we can.

"We were shaking, trying to call police, trying to get somewhere [that would help], but in hindsight there was nothing we could have done. We just had to regather ourselves."

Phoenix and the woman believed to be Ms Woodcock were last seen travelling in a white 1999 Holden Vectra sedan with the registration number YAU56P.

But police said they could also be in a car similar to a blue Subaru Forester, with a broken back window, damaged front and possible registration number YKP08U.

The two cars that police believe could have been used in taking Phoenix Mapham. ( ABC News )

Mr Mapham described his son as an extrovert who was always joyful and loved talking to people.

"He's just a really good kid," he said.

"He likes playing with his ukulele, he likes drawing … he loves playing his go karts, loves bike-riding — we go up to the mountains a lot."

Until now, an amber alert had never been issued for a missing child in the ACT. ( Supplied: Cliff Mapham )

He said the past week had been a rollercoaster of emotions.

"We are at a big loss, [just feeling] anger, desperation," he said.

"We don't know where he is … there's been fears … everyone is trying their best to get him but we feel really hopeless.

"If anyone knows anything, hears anything or sees anything, please call in [to police] and try to bring Phoenix back home."

At a midday press conference, Detective Station Sergeant Harry Hains made a "personal plea" for anyone who knows where Phoenix is, or has been since Thursday, to contact police.

Phoenix's dad said he was an extrovert who loved to talk to people. ( Supplied: Cliff Mapham )

He said he was seriously concerned about Phoenix's physical health and emotional care, and had a message for his mother.

"We need to know that your son is OK," he 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, which could be a hospital, a doctor's surgery, a health facility or police station.

"Please bring him in."

Station Sergeant Hains said amber alerts were not issued lightly, and ACT Policing was working with all state and territory counterparts in their search.

Police asked anyone with information to phone them on (02) 6256 7777 and quote reference number 6300755.