Scientists have battled rough seas and bad weather to place sensors at New Zealand's largest fault line.
The 30 devices at the Hikurangi subduction zone off the East Coast will collect crucial data over the next 12 months.
Geophysicist Laura Wallace says it is a challenging but necessary project, with evidence pointing towards the Hikurangi subduction zone may have been behind the devastating 2016 Kaikoura earthquake.
"There's a lot of things that could go wrong - so we have to be very careful about making sure everything's working properly."
The sensors will measure seabed movement and record earthquakes. Dr Wallace says the devices will capture data on slow-slip quakes, which take weeks to occur.
"We can actually detect pressure changes that are measuring upward or downward movement of the seafloor at a centimetre level."
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Ten instruments were retrieved from last year's voyage.
"We can really better assess what is happening on the subduction zone, and that will feed into our understanding of earthquake and tsunami potential on the plate boundary."
The data will be analysed by PhD students.
Between eight and 10 quakes measuring 7.5 or above have struck the Hikurangi subduction zone in the past 7000 years, scientists estimate. The strongest may have been as strong as 9.0 - releasing 22,000 times more energy than the 2011 Christchurch quake.
Newshub.
