?Maybe a squadron of large migrating birds all relieved themselves at once
That's the standard explanation, and it's spring, which is when the migrants return from Siberia, but the photo (if it's not a stock picture) suggests, as you say, large birds, and the biggest bird in the dense flocks is the godwit, and by the end of their flight (epic, in all truth) I don't think they'd have enough in the tank, as it were, to produce such a heavy bombardment. That stuff looks goose or swan calibre, and they don't fly in big flocks in Auckland. Not to mention the colour: goose turd green is a genuine colour name, by the way, and the picture definitely shows brown jobs.