The very first thing you need to do is speak to the Council and find out in principle whether the block can be subdivided; talk to a planning officer at the Council. Unless it is an exceptionally large block I would be surprised if a single level duplex was viable; work out the area of each unit (allow 2 or 3 bedrooms, living, bathrooms, laundry, garage and circulation) to verify whether a single or two-level building is required. Work out the total building area and try to estimate the building costs using $/ m2 from other similar buildings in the area (a real estate agent might help with that information), and add site works, paving and landscaping. If the figures show a reasonable profit margin (20% +) after costs then once the property has settled (assuming you have to buy the block) the formal application to Council can begin. Apply for Development Consent, Subdivision Certificate and Construction Certificate (I think that's what they're called in NSW).
This is Development 101, a lot to pick up if you haven't done it before so take one step at a time: first approach the Council to see whether a subdivision of the block can be undertaken.