At the eastern end of the island, privately owned Stony Batter has WWII tunnels and gun emplacements that were built in 1941 to defend Auckland’s harbour. The 20-minute walk from the car park leads through private farmland; bring cash for the entry fee. At the time of research, the tunnels were shut until further notice due to safety concerns, but you can still wander around the site.
Stony Batter Historic Reserve
Waiheke Island