Two earthquakes in two minutes, one of them a "strong" 4.7, shook central New Zealand overnight.

GeoNet reported a preliminary magnitude 4.1 quake at 6.13pm, 10 km north of Culverden at a depth of 20km.

The second quake, a stronger magnitude 4.7, struck at 6.14pm, 15 km south-east of Seddon at a depth of 15km.

Initially GeoNet classified the magnitude of the second quake as a 5, but this was later downgraded to a smaller, shallower 4.7.

More than 3000 people, mostly in the lower North Island and upper South Island reported feeling the quakes.

GeoNet says while it's still very unlikely, they estimate the probabilty of a magnitude 7.8 or larger earthquake in the coming year has increased to about five per cent, due to ongoing slow-slip events.

"This is a small increase in the likelihood which is generated by the Kaikoura Earthquake alone," says a statement on the GeoNet website.

"This is about six times greater than it was prior to the Kaikoura Earthquake."

Due to the large extent of slow slip, GeoNet says the adjusted forecast covers a larger region than the standard aftershock area to now include the lower half of the North Island and the upper South Island.

"There are several faults in these areas capable of large quakes, including the subduction zone and crustal faults like those that ruptured during the Kaikoura earthquake."

More earthquakes will occur as part of the Kaikoura earthquake sequence, and we expect aftershocks to continue for months to years, says GeoNet.