Scientists are learning more about the movements of New Zealand's great white sharks - including one young male that crossed the Tasman to cruise Bondi Beach.
Data has recently been downloaded from new acoustic receivers off the east coast of Australia and in the Coral Sea. The receivers detect tagged sharks that swim within 500m of them.
A 2.8m great white known as "Meadsy" tagged near Stewart Island was picked up by a receiver 10km off Bondi Beach in February. Meadsy was recorded on a Stewart Island receiver again in March, having crossed the Tasman in 21 days at an average of 96km a day.
A 3.3m male tagged at Stewart Island was later detected at Chesterfield Reefs, 550km west of New Caledonia.
Principal scientist at the National Institute of Water and Atmospheric Research Dr Malcolm Francis said the new technology enabled more accurate tracking. That could lead to fishing bans in certain areas at certain times - but would also protect people.
"The Australians particularly are putting a lot of money into these acoustic receivers in their water ... to find out where and when the sharks turn up," he said. "That way they can say 'these are high-risk times and places - so not a good time to go surfing or spear fishing', or something like that."
The acoustic technology and earlier "pop-up" tracking has shown that New Zealand great whites travel great distances. The sharks leave in winter for tropical waters near Tonga, New Caledonia and the Great Barrier Reef.
"We used to think that they were just coldwater species and hung around seal colonies and shallow water for most of their lives," Dr Francis said. "But they're like us - they head off to the tropics for their winter holidays."
Stewart Island and the Chathams are the two known "hotspots" for great whites but total numbers are not known. Scientists believe the separate populations may be of distinct genetic stock. Over the past two years, 45 great whites near Stewart Island have been tagged with acoustic technology. Last year the sharks left New Zealand waters at the end of June and started coming back between December and May.
Dr Francis said great whites did not follow the same routes as each other, with the tracks of sharks heading to the tropics being particularly different.
Scientists also tagged a mako shark, "Carol", which swam halfway to Fiji, back to New Zealand, and then back to Fiji - nearly 6400km in 111 days.
Dr Francis said it was not yet clear how great whites navigated so precisely nor why they migrated.
"We suspect they're heading off to find a better food supply. About the time they leave, the young seals at the colonies are getting more agile ... probably harder to catch."
The project is a collaboration between Niwa, the Conservation Department and Auckland University scientists.