An investigation is under way on a serious house blaze in the Auckland suburb of Mount Wellington this morning.

Fire crews attended the Bernard Street address just before 11am. It is not known what caused the fire but emergency services confirmed all people are safe.

"All persons are accounted for. We're still at work extinguishing the fire and a specialist fire investigator is responding to the scene," a spokesman for the fire service said.

"The arrival message was the house was fully involved in fire, so it's fair to assume the house is going to be badly damaged."