A low bringing swells, rain and "copious amounts of rain'' was headed across the country this week, said MetService forecaster Bob McDavitt.
He said Tauranga could be hit with swells of up to three metres over the next three to four days, bringing challenging conditions to the rescue efforts.
"The peak wind and rain is expected Tuesday morning. The wind might ease a bit and come up on Tuesday afternoon, and then ease away.
"But the onshore waves are expected to rise to occasionally 3m during Tuesday. They probably won't ease again until Thursday or even Friday. So that's going to be challenging conditions.''
He said the Tauranga area is expected to see about 30-40mm of rain over the next day or so, which was a third of the monthly average for the area.
Mr Joyce this afternoon told media oil could wash up on beaches for a number of weeks, with the bulk expected to come ashore in Papamoa.
"Even in a successful ocean-based clean up, generally the most people actually pick up at sea is around 10 per cent of the total, the rest of it either disperses naturally or ends up on the beach.''
Although the oil leak from the ship appeared to have stopped, there were fears continuing bad weather could cause the ship to break up on the reef and create further leaks.
However, Mr Joyce said the ship was holding up to high winds better than expected.
"There is no sign yet of it stressing in such a way that would cause a break-up to occur,'' he said.
"The weather we're experiencing out there won't be helping, but that's probably an unexpected upside at this point in that the ship seems to be holding together right now.''
Mr Joyce was less positive about the chances of getting the Rena off the reef in one piece, saying officials had described the salvage as one of the most difficult they had seen.
"Because the ship is rammed so hard on the reef, it was obviously travelling at a good speed when it hit the reef, that is making it pretty challenging.''
Work was continuing to pump oil off the ship and onto the barge Awanuia today, although that was interrupted as the weather created safety concerns.
"If and when they get the clear weather that they need to be able to do this job, it will take between 30 and 40 hours in total to transfer the oil from the ship into the bunkers on the barge.''
Another concern was a number of containers on board that were holding hazardous material, with 11 of the containers holding goods classified as "dangerous''. Another two held goods that could give off flammable gases if they came into contact with water.
All of the containers were sealed and Mr Joyce said officials were continuing to monitor them closely.
There has not yet been a public explanation for the crash, and the ship's captain was being interviewed by investigators.
Whether he had been drinking alcohol was one of the subjects of the investigation, but Mr Joyce said he did not know whether that was the case.
Mr Joyce said the history of the ship was also being considered as part of the investigation, and confirmed that Maritime New Zealand had conducted a check on the ship in the last few weeks.
Mr Joyce said he was not aware of any near-collisions the ship had had, or whether the charts on board had incorrect information.
