The shark is just the second pregnant great white caught off New Zealand's coast and the fishermen whose set-net it drowned in swear it was 7m to 7.3m.
That would put it equal to the biggest great white reliably recorded, which is 7m - although there have been unconfirmed reports of great whites up to 9m.
Thames fisherman Daniel Scott believes the shark was at least 1.8m longer than his 5.1m fishing boat, and Department of Conservation scientist Clint Duffy said that from photographs the shark seemed to be at least 6m.
Mr Duffy sent a photo of the shark to a Waiheke Island fisherman who has fished all round New Zealand, including the sub-Antarctic islands. He said it was easily the biggest great white he had seen.
"It would certainly spoil your scallop dive if you saw it swimming by," said Mr Duffy.
He and National Institute of Water and Atmospheric Research scientist Dr Malcolm Francis made plans to retrieve the fish from its watery grave in the northernmost reaches of the Firth of Thames but decided against it because the carcass would be too decayed.
The fishermen saved two baby sharks by cutting the mother open and holding the youngsters in the water until they swam away.
Marine biologist Peter Crabb said the mother's death was a tragedy that could have been avoided by banning set-netting in near-shore areas.

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