Northland's Cavalli Islands are being surveyed by the Navy for the first time in more than 40 years.
The islands — a popular diving and boating destination off Matauri Bay — were last surveyed in the 1970s by the HMNZS Lachlan.
Now a team of 12 New Zealand Defence Force (NZDF) hydrographers are carrying out a 10-week training survey which will cover more than 100sq km of seabed.
The results will be used to update military charts of the area. The data will also be provided to Land Information New Zealand (LINZ) to update nautical charts used by boaties and commercial vessels.
Lieutenant Commander Dan Wierenga, who leads the Military Hydrographic Group, said NZDF underwater surveying skills were called upon after the 7.8-magnitude Kaikoura earthquake in November 2016.
Eight hydrographers surveyed the sea floor off Kaikoura and mapped shifts caused by the quake, allowing heavy relief to arrive safely by sea while roads and railways were closed.
The last significant update of nautical charts for the Cavallis took place in 1976.
"Cruise ships, yachts, other recreational boats and passenger ships often visit the Cavallis, so it's important that mariners have current charts to refer to," Wierenga said.
LINZ hydrographic survey manager Stuart Caie said the Cavallis had been identified as an area of moderate risk because of the types and numbers of ships visiting the area plus the age of the current charts.
"As part of our long-term programme to improve safety of navigation, we look at the adequacy and accuracy of existing charts and see whether they're still fit for purpose," Caie said.
"Updating the charts for the Cavalli Islands is one of our priorities, and NZDF support for our survey programme has been very valuable."
Data collected by the NZDF would also be used by councils and scientists to better understand seabed habitats and the impact of climate change, for example by helping to produce models for the effect of storms, tsunamis and rising sea levels on coastal communities.
The area being surveyed, 113sq km, is equivalent to 11,200 rugby fields. The job is expected to take until mid-November.
The Cavallis consist of seven islands about 3km offshore. The largest, Motukawanui, is a reserve administered by the Department of Conservation.
The wreck of the Greenpeace flagship Rainbow Warrior, bombed by French agents in 1985, was scuttled at the Cavallis as a dive attraction and artificial reef.