NIWA says the amount of rain which fell on one West Coast glacier between 12am Wednesday and 9am today was more than Christchurch typically gets in a year.
The site of the rainfall was Ivory Glacier near Hokitika, where 652mm of rain fell during that period, outweighing Christchurch's annual average rainfall of 594mm.
NIWA meteorologist Benn Noll said the downpour was due to a phenomena called an "atmospheric river" - a long, narrow region in the atmosphere which transports most of the water vapour outside of the tropics.
Downpours in other elevated regions over the same period were also heavy, with Mt Philistine receiving 508mm, Arthur's Pass 397mm and Mt Cook 351mm.
Mr Noll said the next few weeks should see frequent high pressure systems affecting New Zealand, bringing less rain.
Temperatures are expected to run about average or slightly above average, with another very warm spell coming to eastern areas later next week.