This is number 7 on the list of The Kowhai Walk - The Sundial of Human Development
From the plaque at the sundial:
"There are many different types of sundial, the most commonly seen being the horizontal sundials used to beatify gardens. This sundial is a type of horizontal sundial which uses a series of fixed points located around the circumference of an ellipse. A person is used to produce the shadow by standing on the time of year marked on a figure of eight located in the centre of the ellipse.
This sundial is accurate to within a few minutes and no corrections have to be made for daylight saving. This is because the bronze indicators on the granite columns are moved twice a year. This sundial was constructed to commemorate the 150th anniversary of the Plimmer Family in Wellington".
Give it a go when next you're at the top of the Botanical Gardens - it's fun to see what time it is just by raising your arms up above your head - provided you're standing on the spot...