Maritime New Zealand incident response controller Renny Vandervelde said the large vessel was three-quarters full, with some 2100 containers and about 1700 tonnes of heavy fuel oil on board.
The 23 crew on board were all safe.
A maritime safety inspector was on board assessing the situation, while a 35-strong team from Maritime New Zealand's national oil response unit was working with the Bay of Plenty Regional Council to assess the risk of a spill.
"It's important to stress that there is no oil spill. We have activated this team should the situation deteriorate,'' Mr Vandervelde said.
"The weather currently is very, very good and certainly for the next three days the weather conditions are stable.''
An oil spill was "unlikely at this time''.
The team in Tauranga had acess to a number of trucks, dispersant and booms, and additional equipment could be brought in should it be needed.
While the vessel was stable, fuel in tanks on the port side was being transferred to the starboard side as a precautionary measure.
There had been significant flooding in two cargo holds, and a number of ballast tanks also held water. The water was being pumped out but it what was not yet known if it could all be removed.
Mr Vandervelde said there was no indication of how the ship ran aground and the captain had not yet been spoken to.
"We haven't pursued that at the moment. It's something that we will pursue, but our current focus is having effective oil response capability and reacting should we need to do so.''
The ship's owner will be responsible for its salvage, while Maritime New Zealand's role was to maintain oversight of the oil response and to sign-off the salvage plan.
Salvage advisers would arrive in Mt Maunganui tomorrow to asses the damage and advise how to refloat the vessel.
"The vessel is hard and dry on the reef, so it may take some time. So certainly from current indications, this will be an ongoing and quite a long process.''
One of the options was to unload the containers before attempting to refloat the vessel, but advice from the salvage advisers was needed first.
It was not yet known what the ship was carrying and Maritime New Zealand had requested the cargo manifest.
The ecological impact on the Astrolabe Reef, a popular dive spot 25km northeast of Tauranga, was not yet known.
Mr Vandervelde said he was unaware of any ships of similar size grounding at the reef.
The incident follows a report of failed maintenance standards on the ship three months ago.
In July Rena was detained at the port of Fremantle for 18 hours by the Australian Safety Authority because it was found to have a number of deficiencies including a faulty hatchway, cargo not stowed or secured, and cracked and rusted parts.
"Vessel has not been maintained between surveys'', the Safety Authority report said.
The next update is due at 7am tomorrow morning.
