Fire investigators are set to return to the scene of two overnight blazes in Auckland and Hamilton

Up to 50 firefighters battled flames at a commercial building in Otahuhu in South Auckland, believed to be a vehicle dismantling business.

Crews left the scene about quarter-past-one this morning.

In Hamilton, six appliances and three specialist units were sent to Te Rapa, where two adjoining commercial buildings were on fire.

The cause of both fires remains unclear.

Also overnight in the Auckland suburb of Mt Wellington, five crews were sent to an electrical switchboard fire at a two-storey warehouse.