UPDATED 5.15PM: A 16-month-old boy is believed to have died in a car at Whanganui Hospital.

A police investigation has been launched after the death on Friday.

Acting Central District Crime Manager, Detective Inspector David Kirby says police were notified by the hospital and inquiries are being made to establish the circumstances which led to the death.

He says a post-mortem was carried out on the baby in Wellington on Saturday, and further tests are being carried out

Officers are speaking to the family of the child and hospital staff.

They say they aren't looking for anyone else as part of the investigation.