The Brisbane city council has a map of all the Stormwater drain networks. It was made in 2014 so more drains may have been added. You will be able to figure out how the map works.
If anyone knows if there is a similar map for any other cities, that would be great if you could reply letting me and other fellow explorers know.
I've been surprised how councils have been willing to post their 18 foot death-sewers and not drains. (Probably access and 18-foot death sewers being kind of uninviting)