Is this a sewer cleanout / relief gully or stormwater pipe?

[LorenClayton]

Hello

My house is at the top of a battleaxe lot, with 3 houses in the lot all behind one another. I'm the top house.

Camera work revealed that the kerb adaptor drain runs across my yard to my back neighbour - the middle house. I had a look over the fence and saw this, so I'm hoping this is a stormwater pipe and not sewerage, as it appears to be directly in front of where the drain pipe is. 

It's an unusual situation. The lot has a mild negative grade from the street at the front. I'm the top house, and so stormwater flows from my house to the houses behind me. Or it would, if it wasn't for a great big retention wall behind my back fence. My back yard floods as a consequence. (I've already looked this up and Council won't get involved). 

I'm wondering legally if I can connect my back drain pipes to this kerb drain pipe, which I'm assuming is shared? Or does the owner who installs the pipe "own" the rights to it?

Why though would the house behind me connect to an upward grade kerb? They'd be better off connected to the street behind them, where their drainage would be gravity fed, unless they were unable to get permission from the bottom house, as it would have to go through their yard. Really confused at this stage.

I'd also appreciate some advice re drainage options. Moreton Bay Council won't offer any advice without me disclosing my address, which I don't want to do until I have to. Their website says they don't allow charged systems to kerb for houses with negative grade and that the only option is an engineered rubble pit which won't do much at all.