Auckland Botanical Gardens

While it is not the most beatuful or exotic public garden it is very expansive and diverse for the middle of a sprawling urban city.  It encompasses numersous themes and backs onto an area of bush providing plenty of short bush walks.  One side of the garden is spoiled by traffic noise from the adjacent Southern Motorway but it is so big that you can easily get away from this.  Perfect for families or a lovers stroll and you could easily spend a few hours here. 
http://www.aucklandbotanicgardens.co.nz/