A lunchtime barbecue has gone disastrously wrong for a Brisbane family with their two-storey house going up in flames this afternoon.

Queensland Fire and Rescue's John Longmire said it's believed the fire started from a barbecue that had been left on.

The home was gutted in the blaze that took hold in seconds in Sherwood in the city's south-west about 1.30pm.

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The fire spread across both storeys of the house.

Neighbour Sophia David said they rushed outside.

"There was already people screaming 'fire, fire' and our neighbours said they heard a couple of explosions as well," she said.

"It was all unfolding very quickly."

The family that lives at the home were already outside when the fire broke out on the balcony.

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Firefighters had the blaze under control within an hour and also rescued a cat.

"The house is completely gutted," Longmire said.