"Finally, when they got the spark into it, it just blew up and had a massive little yellow flame, which hit the first fella full in the face. The elderly fella, who I think was in his 80s, got hit as well."
The first man, who St John Ambulance said was 59, suffered partial and full burns to his head, face, neck and arms and was in a serious condition.
He was flown by the Auckland Westpac Rescue Helicopter to Waikato Hospital.
Driessen praised the quick actions of other party-goers, who quickly made sure the fire was out before using hoses to cool off the two victims until help arrived.
He said the incident was "just one of those freaky things that happened and gave him a hell of a fright".
"It was a terrible thing to happen and just one of those unfortunate things."
It was lucky, though, that more people hadn't been injured.
A minute or so earlier, about 12 or 13 people were crowded around the barbecue waiting for it to be lit, but at the time of the explosion they had wandered away, bored.
If it had happened a little bit earlier, they could have been dealing with "multiple casualties", Driessen said.
The elderly man was taken to Thames Hospital by road ambulance. The other man is now in a stable condition.
