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The link I posted previously  points to a long winded article. Much more concise is

https://www.weatherwatch.co.nz/content/did-you-know-there-are-4-different-end-dates-to-winter-2

which contains the following summary:

However there are really four different end dates to Winter – and you can take your pick as to which one you prefer to go by.

So, when does Winter end and Spring start? It all depends on what your definition of ‘winter’ is…

• Astronomical Dates (winter starts on the shortest day, around June 21, spring therefore doesn’t start until the equinox on Sept 23 2002 – this is based on the earth’s rotation around the sun and not the weather here)
• Meteorological Dates (winter starts on June 1, spring starts Sept 1)
• Solar Winter (The 3 months/12 weeks of the year with the least amount of available sunlight with the shortest day of the year in the middle, meaning the Solar Winter runs from early May to early August).
• Mother Nature – For 15 years now WeatherWatch.co.nz has said the reality is that New Zealand really does have a short 2 month summer and a short 2 month winter… the other 8 months are a long drawn out spring and autumn. We see the evidence of this and measure it in the form of animal life cycles, pollen, flowers, buds and grass growth, not to mention current weather patterns too.