An Australian father and son have been rescued off Mt Tongariro after taking a wrong turn.

A statement from police said the 49-year-old man and his 13-year-old son set out yesterday morning to visit 'Mt Doom' in the Tongariro National Park.

It was only when they were near the high point of the Tongariro Alpine Crossing that they were encouraged by other trampers to walk to the end at Ketetahi, rather than return to their car at the Mangatepopo carpark.

However, at Emerald Lakes they took a wrong turn and ended up in the Oturere Valley.

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They realised they were lost when they went past the Oturere hut, but thinking they could see a carpark in the distance they pushed on.

As soon as they got cell phone coverage at 6.40pm they called police saying they were lost in bush between the hut and the Desert Road east of Mt Tongariro.

By then it was dark and cold, and without any headlamps they could not find their way.

Police were able to pinpoint their location using the man's cellphone and the Taupo based Greenlea Rescue Helicopter was sent to the mountain.

The man used his camera flash to indicate his position to the crew with their night vision goggles.

The helicopter was able to hover in the tussock a hundred metres away, then a rescuer scrambled through the bush to find the two, who were cold but otherwise uninjured.

The pair were flown to Mangatepopo.