MetService weather ambassador Bob McDavitt said 3m to 4m-high waves could hit the ship several times an hour but the highest 7.5m waves would be a once-a-day occurrence.
The Rena had survived winds of similar strength in the last week of December, and the worst of the weather would be centred further to the east at Gisborne, he said.
"The Rena has seen stronger winds and waves ... but the Rena is also in a state of decay," Mr McDavitt said.
Divers have been unable to see if the two parts are connected below the surface. Containers on the bow have been lashed down and the more accessible have been fitted with location transponders.
Salvage unit manager Kenny Crawford said salvors would decide on action depending on the conditions if the ship broke up.
"If it breaks in half in 7m waves, are you going to put people in the way of danger? It's an assessment we'll make at that time," Mr Crawford said.
With the bad weather looming, salvors were trying to remove as many containers as possible yesterday before bringing the crane barge Smit Borneo back to port.
Another 17 containers were removed on Thursday, leaving an estimated 881 containers left aboard.
Maritime NZ said work in the No5 hold was stopped after high gas levels were detected. Breathing apparatus are on the vessel and the gas levels will continue to be monitored today.
Spitzer Salvage spokesman Brian Dale said he was not nervous about the wild weather.
"Ourselves and Maritime New Zealand have contingency plans in hand if the ship does start to break up. If it breaks up ... you endeavour to connect to [the Rena] and then stabilise it and stabilise it and hold it," Mr Dale said.
"If it sinks, it sinks. Then it is a clean-up operation. We are not going that far ... we take it minute by minute. We just plan for the worst and hope for the best."
National on-scene commander Alex van Wijngaarden said shore response crews would also have to wait and see. "Our plans are in place and we're ready to mobilise at short notice."
The national oiled wildlife centre at Te Maunga had been downgraded but could be quickly brought into action.
Meanwhile, TVNZ last night reported that the wives of the captain and officer charged in relation to the ship's grounding were denied visitor visas for New Zealand.
