the block we're purchasing (offer accepted) has 149 certificate from Council. It showed the property is identified as being Clause 3 (works beyond 1 meter below the natural ground surface, works by which watertable is likely to be lowered beyond 1 meter below natural surface) on acid sulphate soil planning maps held by Council.

When queried, the council explained the acid sulphate soil and that it is treatable. In case of building a duplex and requirement of excavation, we'd have to get an Environmental Officer or a Geo-tech report. The council said for normal concrete slab and building would be fine.

How expensive is it to treat it? We're planning to build a single story duplex in the next 6-12 months, either normal brick or modular homes. Could it affect it adversely? I have no idea about soil.
Thanks heaps!
How long is a piece of string? If your just building at natural ground level shouldnt be a problem, speak to your builder. Get a geo report done / soil test they should advise as to the extent and what your options are.

Get a geo report done / soil test

When I am thinking about buying a block of land ususally in a new subdivision I ask the vendor via my REA for a copy of geotechnical report as this saves me about 1K.

If the block is part of an older subdivision a report may not be available but no harm asking