Officials said heavy seas had snapped off the stern section of the Greek-owned Rena, which leaked large amounts of oil on becoming stranded in October.
Clean-up teams have been alerted amid fears the ship could leak some of the remaining oil on board.
The accident has been described as New Zealand's worst maritime disaster.
The Rena struck the well-marked Astrolabe Reef off the North Island resort area of Tauranga on 5 October.
'Overnight, the vessel was hit by wave action of up to 6m, heavy winds and heavy seas. That's caused the ship to break into two parts,' Maritime New Zealand spokesman Ross Henderson told the BBC.
'The forward section remains firmly grounded on the reef, but the rear section has broken away.'
Mr Henderson said the two parts of the ship were now 20-30m apart.
'What we do know is there's been a significant release of containers and container debris.
'We don't yet know what amount of oil may have been released, but we are gearing up for an expected release of oil and container debris on shore, and have response teams ready to go once that starts happening.'
Salvage crews have removed more than 1,100 tons of oil from the stricken vessel, but 385 tonnes remain onboard.
Hundreds of tonnes of fuel have already spilled into the sea, killing more than 20,000 seabirds in the area.
The captain and other senior officers face up to 16 charges relating to the wreck.BBC News