Police are taking legal advice on whether to charge a mother who left her baby alone in a van while she went shopping on a steaming hot Christmas Eve in downtown Whangarei.

Police were called to Vine St about 11am on Friday - Christmas Eve - after traffic wardens discovered a baby, believed to be six to eight weeks old, had been left alone in a Nissan Serena people carrier while the mother was shopping.

Temperatures downtown on Friday reached 30C-plus.

Whangarei traffic wardens say they discover on average two young children a week left alone in vehicles in the central city all year round.

On Friday wardens had stayed with the vehicle the baby was in for around six minutes before calling police as they had become concerned for the child's safety.

The mother had returned by the time police arrived. She was spoken to and a decision on what, if any charges, to lay would be made after obtaining legal advice, a police spokesman said.

The mother, who did not want to be named, said she had left the child for "only a few seconds" while she went into a store to pick up a few things, but the shop had been far busier than she expected and she had had to wait.

"She was asleep and all the windows were down. I didn't mean to be so long but the shop was full," she said.

The woman, who said she had five other young children, believed the child was fine, even though she had been crying. She had breast-fed her daughter to calm her down after she returned to the van.

Keith Thompson, boss of Environmental Northland, which does traffic management in Whangarei, said the wardens had heard the baby whimpering then crying and believed it was in distress.

"It's quite concerning for our ladies (to keep discovering babies left in vehicles so often) and we are worried that one day they will find a child in serious trouble, or dead. That's not something they want to see at all and it's something they are really worried about," he said.

While the outside temperature had been about 32C, the temperature inside a vehicle could soon hit about 50C, putting a young life at risk.

"I don't think I'd want to sit in a vehicle in that heat, let alone a young baby like that," Mr Thompson said.

"Any parent should pack up the baby and take it with them to the shop. With children you should always be extra careful."

It is a situation child safety experts warn could be dangerous.

Child, Youth and Family Northern Regional Director Grant Bennett said earlier it was against the law to leave children under 14 without making reasonable provision for their care and supervision.

What was considered "reasonable" also took into account the circumstances under which children were left alone and the length of time they were alone.

"Parents are required to assess all the circumstances and make sure that any child left alone is safe and in no danger. In general babies and young children should never be left alone at home or in a car, or unsupervised," Mr Bennett said.

"Parents need to use their common sense and evaluate the situation if they need to do this for a very short period. In addition to the obvious safety risks, children left in a car can easily get frightened or distressed or, in summer, become over-heated or dehydrated."