Weather: Prepare for incoming 'spring chaos'

New Zealand is being warned to prepare for "spring chaos", which could deliver big temperature swings, bigger lows and spring rainmakers.

"The next 10 weeks ahead are traditionally NZ's most chaotic of the year with winter slowly fading and hints of summer coming in," WeatherWatch warns.

"You can expect a real variety of weather with bigger highs drifting out of Australia towards us but also bigger lows in and around the New Zealand area churning the weather up.

"Mixing large lows with large highs can create big temperature swings as various pressure systems pull airflows from further afield, sometimes mild subtropical northerlies other times cold Southern Ocean southerlies."

WeatherWatch says most of the signs of winter will tend to fade over the next few weeks as flashes of summer appear, but the brief wintry snaps could continue for as far as December.

"Snow and frosts are not exclusive to winter and September is a critical month for growers and farmers in both main islands - sometimes warmer air colliding with cold air can produce snowstorms," WeatherWatch says.

"Even light frosts in September and October can cause millions of dollars of damage to vineyards."