Japanese whaling ships have been warned not to enter New Zealand's Antarctic waters by Prime Minister Helen Clark.
Military planes were patrolling areas of the Southern Ocean for which New Zealand is responsible and would take photos if they see the fleet, she said.
Ms Clark added that the surveillance pictures would be published - but not the exact co-ordinates of the fleet, because of security concerns.
Protesters have temporarily halted the Japanese ships' hunting activities.
Tokyo officials say they want to shake off the activists - some of whom boarded a Japanese vessel last week - before they resume the hunt.
New Zealand has search-and-rescue responsibilities over parts of the Southern Ocean, but does not have territorial waters.
"It's an area that's very difficult to access. If there are problems it's difficult to render assistance," Ms Clark said.
She added: "We won't release co-ordinates for obvious safety-related reasons but we will put information out to the world where we see the fleet."
The Japanese fleet plans to kill about 900 minke whales and 50 fin whales by mid-April as part of what it describes as a scientific research programme.
Other nations and environment groups say the research goals could be achieved using non-lethal methods and call the programme a front for commercial whaling.
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