"What's left standing will be pushed over by a digger," he said. "It was just one huge fireball in the middle of the night - the entire building was engulfed in fire."
Two other buildings on the 2.2ha property were not damaged.
The blaze was not being treated as suspicious.
Owner Denise McGregor said she and her son, his fiancee and their daughter were asleep upstairs when they were woken by the fire alarm.
They escaped without injury but lost most of their possessions. They smashed the window of the family car parked outside so it could be moved away from the flames.
The main architecturally designed building of 900sqm and attached facilities were destroyed. The areas contained the main reception with two nooks, billiard room, staff quarters, commercial laundry, storeroom, commercial kitchen, restaurant, banquet dining room and large lounge.
On the middle level were five large luxury suites with private decks and ensuites, one with a spa bath, while the top floor contained an apartment with lounge, kitchen/dining, deck, bedroom and en suite.
The nearby Stables accommodation building survived the fire.
Reg Walker, 78, and his wife Ngatini, 65, of Manurewa, were staying in the adjacent Stables building when they were woken by the owners about the 2.30am. They had stayed there many times and were devastated by the destruction.
"We were standing out by the road and the great big sparks coming off the fire were absolutely terrifying. The place has been razed to the ground," Reg Walker said.
"It's so beautiful there and to see that, well, it's pretty horrific."
Reinhard Nickel, who owns the nearby Bushland Park Lodge, said guests there were not affected and were back in bed two hours after the drama began.
He feared the surrounding bush could have caught fire, which would have had devastating consequences.
"Our bush is our treasure. If that's gone then everything would be gone and no guests would come any more."
