Large swells and strong winds have caused further damage to the stricken Rena off the Tauranga coast.

Maritime New Zealand says a crack in the container vessel has opened further, but that it may still be connected underwater.

The risk of further damage and loss of containers is expected to continue in heavy weather, MNZ said.

The crack that has opened up - which is now at least one-and-a-half metres - has resulted in the aft section moving more freely than the forward part.

The vessel remains firmly grounded on the Astrolabe Reef, with divers to check the extent of the damage.

The salvage master of salvors Svitzer said he could not "speculate on whether it would spit in two or whether it would move".

"What I can say is that from our naval architect calculations, the ship is still well grounded," said Drew Shannon.

"It is moving, it is lively, but it is not moving off the reef."

At least two containers have washed overboard. More than 900 are still on board.

Meanwhile, more timber and packets of milk powder have washed ashore on Motiti Island.