More reports of damage from last night's earthquake in the South Island are expected to filter in as the morning wears on.

The 7.8 magnitude quake at 9.22pm was centred 100 kilometres northwest of Tuatapere at a depth of 12 kilometres. It is the biggest New Zealand has experienced since the Napier earthquake in 1931. The first aftershock was recorded at 9.41pm and measured 6.1 on the Richter scale. The second struck at 1.50am and measured magnitude 5.9.

The quake sparked a brief tsunami alert, which was later cancelled.

Southland Civil Defence Controller Neil Cruickshank says the initial quake is the worst he has felt. He says it had a rolling motion and lasted the best part of a minute. He expected quite a lot of damage but there have only been reports of minor damage so far. Mr Cruickshank says most of the immediate problems were rectified last night but he expects further issues to arise today.

Seismologists say it was fortunate the main quake was centred in such an unpopulated area near Dusky Sound. GNS scientist, Bill Fry, says it caused such intense shaking because it was fairly shallow. He says more aftershocks can be expected over the next two days but they will become increasingly weak.

Mr Fry says the tsunami risk was real, even although it produced a ripple of just 17 centimetres. He says it had the potential to be much bigger.