Building duplex - need advice for storm water easement

Hi guys,

I am new to this thread and wanted some advice on a stormwater easement.

I am buying a large block of vacant land that comes under Campbelltown council, and the current block has no storm water easement.

The block slopes away from the road, so from the road if we look at the block it's slope would be approx over 1m to 1.5m as it reaches the back fence line. It also slopes again from the right back fence line towards the left for another 0.5 to 1m at the rear of the block.

Our left side neighbours don't have an easement. Our Right side neighbours do, but that would be slightly elevated as compared to our block slopes. Behind our back fence line, we have townhouses but they don't have any easement either.

We have been to a few builders, met with council but haven't been able to find the best solution.

We have been told that a pump out system isn't allowed for dual occupancy dwellings. We think we should be able to tap into our Right side neighbours but aren't 100% sure.

The other options we have been told are absorption pit, and charged lines but what are the chances of the council agreeing to this?

Could anyone suggest who would be best to consult to find out the solutions? i.e would a Hydraulic engineer be helpful? Would they look at the council rules and advise accordingly? do they provide costs for the proposed solutions as well?

What other options can we propose to council for stormwater easement?

Thanks in advance,
Abhi