What gull is this?

Last week I was in Mason Bay, Stewart Island, New Zealand. The wind was really strong off the ocean and conditions were perfect for vagrants. I was optimistic for some weird penguin or albatross, but what I found was a seagull.  At first I thought it was a pacific gull (Larus pacificus) which would have been really cool because according to The Hand Guide to the Birds of New Zealand there's only one record of pacific gull in New Zealand. However, I'm a little confounded because the gull I photographed in Mason Bay does not fit neatly into the Hand Guide's description of a pacific gull. For one, the pacific gull is supposed to have red markings on both the upper and lower mandibles, including extensive red on the lower mandible, and the bird I found only has a small spot of red on the lower mandible. Furthermore, the pacific gull is supposed to have a black subterminal band across its tail, followed by a white terminal band. The gull I photographed doesn't seem to have a terminal white band, or its white band is very narrow, much narrower than the band depicted in the Hand Guide. Finally, the leading edge of the wings of the bird I photographed are white, while the Hand Guide depicts them as black in the pacific gull. So, this bird looks superficially like a pacific gull, but doesn't fit exactly into the field guide's description. I know gulls are tricky to identify, but I'm travelling without access to any additional resources. One thing is for sure, this isn't any of New Zealand's regular gulls. Does anyone know what kind of gull this is? Please let me know.

Update: So it's just a black-backed gull, a very common gull in New Zealand. It's tricky, though, because it seems to have retained a few juvenile characteristics, such as the black tail band, into an otherwise adult plumage. Discussion of the bird by New Zealand birding experts can be found here.

This gull wasn't the only good find at Mason Bay, we also found this other bird, which we don't need any help identifying.