It's either a non-breeding adult or a juvenile, but that's just the size WWBTs are. White-fronted Terns are a medium-sized tern, quite a bit smaller than Red-billed Gulls (for example); for Europeans, WFTs are just a tad bigger than Common and Arctic Terns.
If you look at the other photos under NZ Wildlife, the Caspian Terns are gigantic (relatively speaking), WFTs medium-sized, WWBTs and Black-fronted Terns and Black Terns all similar in size to one another (although I think the Black Tern is smaller than the other two Chlidonias species pictured), and Little Terns are even smaller than those three. [Edit: I forgot about the Sooty Tern, which is a little bit bigger than the WFTs]
One of the photos shows the Black Tern alongside WFTs, and another shows the Little Tern alongside WTFs, Caspian Terns, and gulls.
Of the terns I have seen this year, only the Common Tern was not photographed (I only saw it once, and I didn't have my camera with me that time). There was an Arctic Tern around as well, but I haven't seen that one yet.
