Teen critical after 'flammable liquid' sparks fire pit explosion north of Newcastle

Five people have been injured - including a teenage boy who is in a critical condition - following an explosion at a home north of Newcastle.

Police believe a flammable liquid was thrown into a fire pit on a patio of the Corlette home about 5pm on Sunday, sparking an explosion.

A 16-year-old boy suffered burns to his face, arms and torso and is in a critical condition at Sydney's Royal North Shore Hospital.

A father and a male relative, both in their 40s, were treated at the scene for burns to their legs and torso and are in a serious but stable condition at the same hospital.

Two others, a man and a woman, also in their 40s, were treated at the scene for burns to their face and torso and were taken to John Hunter Hospital in Newcastle in a stable condition.

Police have established a crime scene at the house and are investigating the incident.

With AAP.