One of my favourite things about Auckland are the ~50 volcanoes scattered across the city. There are also rather excitingly a number of lava caves, through which molten lava traveled before finally settling downhill / in the sea somewhere. There are three main ones I know about, easily accessible if not huge on Rangitoto, huge but very inaccessible (DOC controlled) in Wiri, and large and occasionally accessible in Three Kings, which I recently visited through the University of Auckland.
Stewart’s Cave in Three Kings is located in someone’s back garden! It’s about 180m in length, up to 14m wide and 8m high in places. It was discovered in 1850 by R. Taylor but is named after James Stewart who developed a comprehensive map of the cave in 1869.Here’s the slightly man modified cave entrance, with a gate to try and control access for safety reasons.