Scientists are questioning whether a magnitude 5.5 weekend quake did indeed strike on New Zealand's most dangerous mainland fault.

If the event - recorded near Big Bay in the far north of Fiordland at 3:24am on Sunday - did actually rupture on the Alpine Fault, it would have been the most significant shake observed on the South Island's high-risk, 600km-long quake-maker in nearly two decades.

Only, seismologists still weren't so sure.

"The reason we are putting a bit of ambiguity around whether the earthquake happened on the Alpine Fault is just because of the quality of our earthquake location data