Workers pumped about 21 tons of oil Sunday night from the cargo ship Rena, stranded on a reef off Tauranga, The New Zealand Herald reported.
Salvage experts said rough weather and choppy seas were expected to hit the reef sometime Monday.
That could tear the broken and "dying" vessel from its position on the reef, releasing more oil into the sea, Maritime New Zealand salvage chief Bruce Anderson said.
"If the vessel falls off the reef it could puncture a tank," he said. "You don't know what's going to happen if it falls off the reef."
The ship was moving on the reef even in low seas and could break up in high seas, salvage officials said.
"That ship is very, very sick," salvage crew spokesman Matt Watson said. "She is fractured. She is broken. She is on her knees."
"That massive gaping crack down the side ... it is quite obvious that if strong winds and strong rain come again that vessel could break up."
About 350 tons of oil already had leaked from the Rena and washed up on local beaches, the Herald reported.