The Rena has split into two pieces, which are now up to 30m apart, after being pounded by bad weather this weekend.
Swells of up to 6m hit the grounded container ship overnight.
The stern section of the vessel had broken away and was moving significantly but remained on the reef, Maritime New Zealand said this morning.
The front section remains in its original position on Astrolabe Reef, off the Tauranga coast.
A large amount of containers and container debris have fallen from the ship overnight.
One witness, Warwick Roberts, said the rear section was sliding on the reef.
“Stern has reared up and centre section is not visible . Large breaking waves observed on bow.”
The National Response Team has been activated to respond to the potential release of oil from the ship and treat any affected wildlife.
A three nautical-mile exclusion zone around the ship might also be extended as a result of debris in the sea.
Weather conditions continued to be poor, with severe weather expected to pound the area for the next three to four days.
Officials had been on standby for the worst weather to hit the ship since it grounded on the reef on October 5.
Swells of up to 7.5m were predicted for this weekend and salvors worked hard on Friday to remove as many containers as possible and secure others before leaving the reef in preparation for the bad weather.
Just last week the ship was pounded by bad weather, causing it to break into two pieces which remained firmly on the reef.
Divers were unable to confirm whether it had broken in two under the waterline when they checked the damage.
Photo courtesy of NZHerald.co.nz and Maritime NZ