They had cleared ashes from the fireplace after the lesson, and then left them on a barbecue on a back porch. It is thought the hose of the gas bottle caught fire and exploded.
"About 5.30am, I heard this woosh and I thought there was a balloon going up the air. I flew out of bed on to my balcony, but didn't think anything more."
Herd manager Gary Sullivan, who had just finished milking and also heard the "woosh", jumped on his quad bike, raced over to the house and woke up the family before calling the fire brigade, said Ms Mebus.
"He's the real hero," she said.
Mr Sullivan used a hose on the fire, and Mr McKenna said his quick thinking helped contain the flames until the brigade were able to travel the 25km from Featherston to extinguish it.
Ms Mebus said the fire brigade, neighbours Neville and Annie Davies and Mr Sullivan had done an amazing job.
It was lucky no one was hurt and a reminder that ashes and embers take 10 days to cool and should be put in water. "I'm just grateful that nothing happened to them. I don't care about the rest, I'm insured, and even if I have to lose my excess, it's not worth lives."