Niwa provide a great explaination behind the tornado in Waikato on October 29, 2018. You can read more about it here
The why behind the Waikato tornado🌪️— NIWA Weather (@NiwaWeather) October 29, 2018
1️⃣ Significant upper atmosphere disturbance.
2️⃣ Much colder (🔵) than usual pocket of air aloft.
3️⃣ Time of year - increasing spring sun angle leads to more surface heating. Lends itself to more atmospheric instability. pic.twitter.com/llHbaKUiMe