Early yesterday morning, the temperature in the city rose to 18 degC at 4am and went on to reach the high for the day of 18.1 degC at 6am.
MetService meteorologist John Law said it may have made it a little unpleasant for those used to sleeping in cooler conditions.
He said the unusual phenomenon was caused by "the Foehn effect" — when a moist northwesterly wind dumps much of its moisture on the west side of the Southern Alps before drying and warming up as it heads towards the east coast.
He predicted there would be more warm nights like this to come over the next few months.
He said by 7am, the wind had turned more southerly, rain started falling and the temperature plummeted to about 10degC where it stayed for the rest of yesterday.
The southerly change also brought a light dusting of snow to the higher parts of inland Otago and Southland.
"It’s amazing to think it doesn’t require much to change the weather significantly, particularly this time of year when we can see a real change in the temperatures throughout the day."
Mr Law said the weather was expected to be calm and sunny around Otago and Southland again today, with some light frosts inland this morning.
But he advised people to make the most of it — the rest of the week would be filled with showers and rain.
On Thursday and Friday, a strengthening moist north to northwesterly flow is expected to spread over southern New Zealand.
Fronts embedded within this flow are likely to bring possible periods of heavy rain, particularly to the west.
However, there is some uncertainty in the computer models regarding the timing and movement of these fronts.
"Despite the rain, looking towards the end of this week, we’ve still got some pretty warm temperatures to come — 21degC to 22degC," Mr Law said.
