Queenstown residents were shaken awake early on Sunday morning by a strong quake.
The 5.5 shake struck at 3:24am at only 5km deep, 40km northeast of Milford Sound. GeoNet reported it appears to have struck on the Alpine Fault.
More than 600 people reported feeling it on the GeoNet website.
"This quake was largely felt in the Queenstown and Wanaka areas," GeoNet said on its Twitter account.
The big quake was followed by a series of smaller aftershocks, measuring between 2.5 and 3.9.
"This fault system has the potential for larger events and we would like to make sure that you are prepared for a large earthquake at all times," GeoNet said, urging locals to follow Civil Defence and EQC advice.
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Scientists last year warned the Alpine Fault was due for a strong quake measuring 8-plus, which are estimated to happen every 300 years. The last one that big was in the early 1700s.
"There will be a similar earthquake soon... Whatever we can learn about this fault and how it moves is, to some extent, helping us understand and prepare for the next great earthquake," researchers wrote in the New Zealand Journal of Geology and Geophysics in September, in a special issue devoted to the Alpine Fault.
That issue was due to coincide with the 300th anniversary in 2017, but research into the Kaikoura quake - one of the most complex ever recorded - took priority.
"The surprises and insights we gained from witnessing a major surface-rupturing earthquake in modern times have made us even more motivated to compile and present current research on the Alpine Fault."
A 400 km rupture of the fault is expected to happen within our lifetime, which will have a broad impact on much of the South Island's critical infrastructure, Project AF8 lead scientist Dr Caroline Orchiston said in May last year.