Was that a -7C ground frost then ? Wunderground has -2C for the 6am reading which fits with my own -1.8C low.
I presume it would've been a ground frost - a -7C air minimum would probably be a record for September. I heard the frost mentioned on Newstalk ZB's weather report.
Snow fell to around 400m in the west and south of the basin.
The interesting thing about this latest icy blast was that snow levels lowered and snowfall intensities seemed to increase the further north one went: 400m near Alexandra, 300m on Port Hills, sleet in Amberley (only just above sea-level), and snow to sea-level at Kaikoura (according to Malborough paper online, via Stuff). The Kaikoura area would've got the strongest impact - probably very unusual to get so much snow to low levels there so late in the season.
I believe there was snow on both the Rimutaka and Desert Roads later yesterday and last night, but not enough to close them.
My parents, having heard that Arthurs and Porters Passes were very dodgy, even if not officially closed, decided to return to Canterbury via the Lewis Pass instead. There was plenty of snow (settling from near Maruia Hot Pools to about the Hawarden area), but not enough to seriously disrupt transport. Apparently Hanmer township got a fair bit more than the state highway not too far aawy. Banks Peninsula got a lighter coating, settling probably to about 300-400 metres.