Eight people suffered burns when an Australia Day bonfire exploded at Ourimbah on the New South Wales central coast.

Police say about 100 people were at the fire in the paddock of a rural property last night at about 8:30pm (AEDT).

Officers believe a large amount of fuel was poured on the fire, sparking the explosion.

Eight people between the ages of 14 and 20 were treated for burns - three are in a serious condition.

Five of the injured are expected to be treated at the burns unit at Sydney's Royal North Shore Hospital.

Police are investigating.