A woman is in serious condition after an explosion in a campervan at a camping ground near Queenstown.

An ambulance is transporting the woman and another woman, who is in moderate condition, from the Lake Hayes Motorhome Reserve to Lakes Hospital in Queenstown, St John southern communications shift manager Andy Gray said.

Mr Gray believed a gas cylinder inside the campervan had caused the explosion.

St John was called to the incident at 7pm.

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The Fire Service was called about the explosion at 7.05pm, Southern Fire Service communications shift manager Lyn Crossom said.

Fire engines went from Frankton and Arrowtown. A fire engine from Queenstown also went to take photos with a thermal imaging camera.

Ms Crossom confirmed a gas cylinder inside the campervan appeared to have caused the explosion.

The campervan wasn't on fire when the Fire Service arrived.

Ms Crossom said the Fire Service would not be doing any further investigations.