Drain you're talking about would appear to be a continuation of maze, although I don't really know where it 'ends' as such - if you shoot me exact coordinates I can tell you 100% whether it'll be walkable and what system it belongs to.
There aren't really any proper bunkers in Melb, underground at least. There have been many rumours of bunkers at Royal Park, Dights Falls and Mt Evelyn but these have never been substantiated and would be pretty much impossible for an average explorer to get into if they did.
Hiding in a drain might help you survive the initial blast, but as the water infrastructure (sewerage and mains) takes a hit, you'll probably find a lot of nasty irradiated debris floating down there...