The Rena was carrying about 1700 tonnes of heavy fuel oil and about 70 tonnes of marine diesel when it ran aground this week.
None of the fuel has been removed from the vessel and it is still not known how much oil has leaked.
The leak appears to be coming from a 100 tonne volume fuel tank. Authorities had said the leak may have been coming from damaged pipework.
Department of Conservation (DOC) and National Oiled Wildlife Response Team members are monitoring the area for injured wildlife and checking reports from the public, who have been told not to handle any animals themselves.
Anyone who sees what they think may be oil on a shoreline should ring 0800 OIL SPILL (0800 645 774).
More staff and volunteers are on standby in case the response needs to be expanded. DOC staff from across the North Island have been put on standby and a number of boats are ready to go.
The reef - about 4 nautical miles north of Motiti Island and 12 nautical miles off the coast - is home to wildlife including little blue penguins, seals and petrels.
Going to get worse before it gets better
Minister of Transport Steven Joyce said today the situation with the stricken ship did not look good.
"The situation with the oil is going to get worse before it gets better, I think there's no getting around that fact.''
Mr Joyce said the top four teams in the world have scrambled to help with the operation.
He said 100 staff are registered with the Incident Control Centre and there has been excellent support from international partners.
"It's a very complex and very difficult operation and the reality is I think you wouldn't choose to start from the position that these good people have found themselves in.''
Mr Joyce said two oil recovery vessels have been sent from Auckland and Picton.
Tauranga City Mayor Stuart Crosby shared the minister's concerns this afternoon.
"It's going to be a huge exercise and I think we need to brace ourselves for quite a significant event.''
Concerns for crew
There are fears for the safety of the 23 crew members on board the stricken cargo ship.
The Filipino crew have remained on board the 236m ship since it struck the reef early on Wednesday.
Media were taken to look at the oil spill today by the Tauranga Coastguard and the Western Bay of Plenty harbourmaster.
Their boat stayed on the edge of a 1000m exclusion zone set up around the vessel, monitored by Maritime New Zealand patrols.
While there appeared to be little activity aboard the Rena, it is understood the crew are working night and day to pump water out of Rena's pierced hull and transfer oil to safer parts of the ship.
Duty skipper Pete Donaldson said the salvage operation and what would happen to the crew was at the forefront of his mind.
"The last thing I want is for them to abandon ship and leave it to the elements,'' he said.
"As far as the Tauranga Coastguard's role, we are concerned.
"My concern as a coastguard duty skipper is there is 23 guys out there and if the weather plays up, what is going to happen to them?''
Bad weather and large swells are forecast to hit Western Bay shores next week.
Previous rescues have involved a rescue helicopter winching people from vessels and onto Coastguard boats.
"That might be possible in the middle of the day with calm conditions but not with 30 - 40 knot winds blowing us around,'' Mr Donaldson said.
Harbourmaster Jennifer Roberts said she felt for the crew.
"I have a bit of empathy for the guys on the ship because I'm a seafarer _ driving ships like that is what we do. I understand how they feel.''
She said two boats seen close to the ship were where salvage experts were diving from to assess the damage.
