Senior Sergeant Brent Murray said it appeared Mr Fisher had been called to the house by the group of teenagers there, mostly young women, after an unwelcome male visitor arrived.
The group were being supervised by adults and were not thought to be drinking alcohol, said Mr Murray.
An argument erupted and Haira then allegedly assaulted Mr Fisher, though police would not say whether a weapon had been used.
The assault comes less than 24 hours after Mr Fisher led the charge to extinguish a garage fire on Oxford St about 3am on Saturday.
Martinborough fire chief Garry Jackson, who was on his way to Wellington Hospital yesterday, said Mr Fisher had done a brilliant job as officer-in-charge at the fire.
"His conduct at the scene had been outstanding and he was feeling very pleased with himself."
Mr Jackson said Mr Fisher was a "top man" when it came to fire rescues from cars and had led competition-winning extraction teams.
He said that, although Mr Fisher worked in Wellington on weekdays, he always made it back to Martinborough for the Tuesday night practices and returned on weekends too.
Mr Fisher has been with the Martinborough fire brigade for 22 years.
Wellington Hospital confirmed late yesterday Mr Fisher was in intensive care in a serious condition.
