Police have launched an internal investigation into why an eight-year-old Aboriginal boy was left unattended in a police paddy wagon allegedly for two hours in northern New South Wales.

A spokeswoman for NSW Police said an internal police investigation was underway into the circumstances surrounding how the boy came to be left unattended in the police vehicle at Coraki.

The boy's mother, Jane Williams, said she was at work when she got a call that her son had been picked up by police.

Ms Williams said she received a call at about 1:00pm on April 13 and went to collect up her son.

She said when she got to the police station the police officer there at first did not know where the boy was, but after a call to a colleague the officer let her son out of a paddy wagon parked at the station.

Ms Williams said her son, Mundhra, had allegedly been held in the wagon for at least two hours and was struggling for air, sweating profusely and crying when she got to him.

"I was in shock, I couldn't believe that was happening," she said.

A police statement said the child was not injured and "local management action has been taken".

"As the internal inquiry is ongoing, it's inappropriate to comment further," the statement said.

The NSW ombudsman will be overseeing the inquiry.

Son was throwing rocks at council car: mother

Ms Williams said the boy had not had any water while he was in the wagon.

"He couldn't even breathe properly," she said.

"He found a little hole at the bottom of where you sit in the paddy wagon and felt the cool breeze through there. He had to lay his head on the ground, on the dirt, just to feel that little bit of cool breeze.

"My son is a dark-skinned little boy. His face was red, his body felt hot.

Coraki police station where an eight-year-old boy was allegedly kept in a police paddy wagon for two hours. ( ABC News: Eloise Farrow-Smith )

"By the time the ambulance arrived they checked him over, they've done all the observations and felt that he was OK to go home."

Ms Williams said she called for legal advice soon after finding her son.

She said her son was picked up with his nephew for throwing rocks at a council car at the Box Ridge reserve, but the other boy was dropped off at home soon after.

Ms Williams said Mundhra was being watched by his aunty on the reserve while she was at work.

When she was first notified she went to her sister's house and to her own home before going to the police station.

Ms Williams said when she arrived the police officer at the station told her he thought he was dropped off at his aunt's house and had to call another police officer to double check.

She said he then dropped his phone and then went to the paddy wagon outside.

Ms Williams said the police officer apologised to her, but she was not happy that he remains on duty, pending the results of the investigation.

She said a local police inspector from Lismore had spoken to her.