Other teams would be assessing vulnerable shorelines and looking for affected wildlife to be taken to a make-shift bird cleaning and rehabilitation centre at Te Maunga.
The heavy fuel oil appears to be leaking from pipes on the 236m Rena, rather than one of the fuel tanks.
Maritime New Zealand national on-scene commander Rob Service said the damage to the ship was extensive and as a result it was impossible to be sure where the leak was coming from, if one of the fuel tanks on board had been breached or how the leak could be stopped.
"It's difficult to know much about it due to the hazards of operating in that location and environment. Half of the vessel is firmly grounded. Stopping the oil leaking is very difficult, there's a lot of damage to the ship."
Mr Service was not sure how much of the estimated 1500 tonnes of oil on board had already leaked into the ocean but said it was doing so intermittently, with a major spill overnight on Wednesday and another yesterday.
"We don't know how much oil [has leaked out] because it's been intermittent and the measurement of changes in oil content on [the] ship is difficult because of damage," he said.
"This is a serious incident and we're taking it seriously."
The ship's crew remain on board and are working to transfer oil to undamaged areas of the ship.
"As you can imagine they're under a great deal of stress and pressure," Mr Service said.
There was no sign the containers, some of which contain dangerous chemicals, on board had moved or that any of them were insecure, Mr Service said.
Among the substances on board is ferrosilicon which can produce large amounts of extremely flammable hydrogen gas when exposed to water.
International wreck salvage firm Svitzer, from Denmark, has been chosen for the operation on the stricken ship. Representatives are now on board Rena.
Two investigators from the Transport Accident Investigation Commission have been on board Rena interviewing crew as part of a parallel investigation to that being conducted by Maritime New Zealand.
The ship has been deemed hazardous and Mr Service said he had 400 oil spill responders to call on, the majority from regional councils. Others have also come from overseas. The Bay of Plenty Times understands further oil spill response equipment, including booms, are coming from around the country.
So far four birds have been found dead and an oiled wildlife response centre has been set up in Tauranga. Four other birds have been found covered in oil.
Oiled wildlife responder Kerri Morgan, from Massey University, said the area was well known for bird life and fur seals.
Ms Morgan said about 20 responders were expected to be settled on the Motiti Island overnight, ready to respond at first light today.
The team on the island will include veterinarians.
Environmental adviser Simon Coubrough said the effect the oil may be having on marine life including scallops, kina, paua and rock lobsters was something the team planned to, "get a handle on quite rapidly" with testing underway at the site and on the mainland.
The regional harbour master has reminded vessels without authorisation to remain outside the exclusion zone surrounding the grounded cargo ship.
The 1km (0.62 mile) exclusion zone remained in effect, and urged people to also avoid the area affected by the oil slick.
See Friday's Bay of Plenty Times for 6 pages of extensive coverage on this oil spil crisis.
