Red-billed or black-billed gull

[emmyhinkloff]

Let me preface this by saying I'm incredibly new to birding, so am aware this may be a stupid question. I'm also aware that this is most likely a red-billed gull and I'm way overthinking this, but figured I'll see a lot of gulls in my life and might as well get some pointers nonetheless.
I took a photo of these gulls not expecting anything - in fact I mostly just point my camera at a group of birds and then try identify them later using Merlin, Paul Scofield and Brent Stephenson's Birds of New Zealand: A Photographic Guide, and The Field Guide to the Birds of New Zealand. The only reason I'm really posting here is because the bird resembles an adult red-billed gull in most ways bar the beak and legs. I understand the red-billed gull has this colour beak and legs when juvenile/immature, however there are immature red-billed gulls in the image, and their eyes are all brown/black, with their plumage showing either speckling or brown secondaries (? the feathers above the vent, indicated with green arrow). The adult red-billed gulls all show coral beaks and legs, so why does this ostensibly red-billed adult not? Is it close to fully mature, where the eyes have turned white but the legs and beak haven't yet?

[Mike Bickerdike]

This is an immature red-billed gull. First plumage juvenile red-billed gulls show the brown-grey markings on the secondaries, as circled in green in your photo, but these marks go when immature birds get into their second plumage, when the bill also starts to turn redder. Black-billed gulls have a finer bill than red-billed, and lighter gray back/wings.