Sewer/ Drain pipe issue complicated by shared with neighbours

Hi,
we have a sewer drain pipe issue after an overflow in the ensuite bathroom from showering. For a couple of weeks now we can smell something musky when we run our taps but didn't quite know what the issue was.
The plumber didn't use a camera but because he couldn't pass his hose through he thinks it's broken pipe that is causing the issue. He wants to fix it asap at a cost of 3-4k verbal. it will cost more for us as we will need to re-do landscaping.
The blockage is partially cleared as he claimed.
We have decided to get a second/third opinion.

Location: Brisbane- BCC. (my location says NSW where we moved from, I just never got to changing it).

My questions
1) How would an experienced plumber diagnose the issue? Should be with a plumber?
2) The sewer pipe is shared by 3 other neighbours uphill from us. It's called a "network pipe" as legend on Dial before you dig. Would this be a water authority/ council issue then? (Yes, I will be calling them but just want to know if anyone has similar experiences).
3) If water authority says not their problem, then would the other 3 neighbours have shared responsibility to unblock the pipe even if it's blocked at our end? How would one go about doing that properly? I assume getting it properly diagnosed, quotes and all, and then having a friendly chat?
4) Anyone with experience relining pipes?
5) Recommendations for plumbers that will service North Brisbane.

Attached is a cut up of diagram from dial before you dig. The network pipe which I think is the issue runs in the backyard of our house, N1 and N2. Our house is the lowest where it then connects to the street (S). Big circles are manholes.

Much thanks!

 

In the first instance, put it back to the authority.

What did the plumber advise? Yours 100%, shared or the board?

 

Plumber doesn't/ or wouldn't know when he came to look. he must have thought it's just our pipes running down to the main sewer in the street (S). Like I said he didn't even use a camera, just told us he's sure it's broken as he can't his hose through and using the camera will just waste our money. I didn't think about it until I remembered the DDYD diagram I pulled up when we bought the house 10 mins before my post.

 

Interested to know how things worked out for you @Lastgeckoonmars

My neighbour had a blocked drain and called QUU 'the network operators' who sent an inspector and found that the shared pipe could be broken at my property (no symptoms in my yard). The neighbour's problem was resolved (don't know how), my side I am still thinking what needs to be done?

 

Hi Damz! We actually made an error thinking it's a shared pipe but according to QLD utilities it's not so we paid for the whole lot.