Mr Anderson said there were another 220 tonnes in two settling tanks near the engine room, and the rear starboard tank holding 350 tonnes was underwater.
He said thermal imaging has made "us confident that the oil is still in the tanks, and there has been no rupture to the one underwater.''
Mr Anderson said the aft section of the vessel was in more than 50 metres of water but if there was no more bad weather the Rena would rest on the reef.
Workers will either get into the hull through a hatch or by cutting their way in.
Oil from Rena has been spotted as far away as Whale Island, Whakatane.
MNZ director Catherine Taylor said today they were waiting to confirm reports of the oil on the island, 9.6km from Whakatane.
Moutohora, or Whale Island as it's commonly known as, is considered to be a sanctuary from native flora and fauna, free from goats, rats, cats and rabbits which previously devastated native plants and animals. The island is also recognised as a great fishing and diving destination for tourist operators.
MNZ today admitted it should have acted quicker in organising volunteers to help with the cleanup from the stricken Rena.
Oil is coating nearby beaches and at a media briefing this morning Ms Taylor said about 2000 people had volunteered to help clean up.
"In any situation like this there are things that you learn from it and organising the volunteers was something which we have come to a bit later than we probably should have,'' she said.
Ms Taylor thanked the hundreds of volunteers helping with the cleanup effort and said planning was moving further east to Whakatane.
She encouraged people to keep volunteering and said although they may not have been called on today, they would be needed as the cleanup continued.
The oil washing up on beaches is so toxic that one young woman helping with the clean up had used her cellphone and wiped the numbers off the faceplate, Ms Taylor said.
It was vital that oil from the beaches did not spread to "clean'' areas, she added.
"At Papamoa Beach, in the carpark, you could see large globules of oil that had come off the beach,'' she said.
"We don't want that contamination because what that means is that the oil will then go back into the system, through the stormwater drains, out to sea and be recirculated.''
People needed to wear protective gear and make sure their shoes were covered so they could discard the covering as they left the beach rather than traipsing it into the clean areas.
One Tauranga hotel had oil in its carpet, some people had it on their clothing "and you cannot get it out'', Ms Taylor said.
"It's a very serious issue. It's very toxic. Please keep the clean areas clean and listen to our advice,'' she said.
