"There was a domino effect. One branch let go and it hit another - and that one broke and hit another - and that broke."
The removal of the broken branches meant the tree, which has long been a landmark given its great spread of long branches, would have a "gaping hole" in the middle, meaning extra stress would go on to other foundation branches, Mr Billing said.
Even trimming back the foliage which overhangs both the northern end of the square as well as much of the road would also create extra stress as the species was susceptible to "sunburn" and exposed branches would drop sap.
"It comes down to being a safety issue," Mr Billing said.
"It is in a popular area where there are people strolling by and cars parking underneath."
He and Napier City Council chief executive Neil Taylor met yesterday to discuss the future of the tree. The final decision on its fate had not been made yet.
The Moreton Bay fig, which is thought to have been planted just over 100 years ago, is the largest tree growing in any of the city's reserves. The trunk has a girth of 5.2m and the canopy spread is 32m in diameter.
"It's just unfortunate this has happened but it is like any tree - none last for ever, they all have their time."
Napier couple Ion and Sylvia Brown were passing when contractors started removing the damaged limbs just after 9am.
"It would be sad to see it go but hey, it does come down to safety," Mr Brown said.
"This was the one tree where children could actually play on its roots because a big lump of them are above ground. It's a real old landmark."
Six years ago the tree lost a main branch after it was struck by a passing truck, and on another occasion a campervan became wedged under one of the low branches when the occupants parked up.
They managed to free it by deflating its tyres.