It would take "another Cyclone Bola" to prise the Rena's dislocated bow from its tight grasp on the Astrolabe Reef, the MetService says.
Nearly 400 containers remain inside the split and stricken container ship's bow - and two had to be yanked from it by tug boats yesterday after a crane barge could not get close enough.
Inspecting the Rena's stern, mostly submerged against the reef after snapping from the bow amid rough seas on January 8, was also proving a difficult and dangerous job with sea surges and jagged steel threatening divers.
A rancid smell from rotting container cargo also hung over the wreck, while tests were being undertaken to measure the toxicity of the water surrounding the ship's aft section.
Maritime New Zealand salvage unit manager Kenny Crawford said no buoyancy remained in the bow and questioned whether it could be swept from the reef.
MetService weather ambassador Bob McDavitt said the Rena was now slowly becoming part of the reef, where it had sat precariously since running aground off the coast of Mt Maunganui on October 5.
The only swells powerful enough to loosen the bow's grip would be a chain of low pressure systems that would bring a maximum wave height, he said.
"The last time we saw anything like that was in 1988, with Cyclone Bola. But I don't think even that would dislodge it. It's sitting on that rock pretty comfortably."
Divers were yet to give salvors a complete picture of the wreck - but Mr Crawford said the ship's breaking in two would make the operation longer and more dangerous.
"How do you get this ship off? Do you cut it and take it away, do you try getting air bags underneath it, we don't know - it could be any number of things," he said.
"It's not impossible - but it's made it more difficult for them."
While some oil continued to leak from the wreck, the amount still trapped in pockets was now estimated to be in the tens of tonnes at the most, compared with the 360 tonnes that leaked in the major mid-October spill.
Meanwhile, container recovery company Braemar Howells had retrieved 64 containers since the Rena broke apart, including five badly-damaged containers unloaded from a barge on Wednesday night.
At Waihi, plywood and sheets of board had come ashore, while at nearby Bowentown Heads, enough latex gloves had washed up to fill about 50 bags.
Debris was still being collected from White Island and Whale Island.
Large numbers of dead birds had been found this week but only one, found at Rabbit Island, had been oiled.
About 50 birds have been sent to Massey University in Palmerston North to check whether they died from natural causes, as was common at this time of year for juvenile penguins.