Buying off the plan means that the property is being advertised for sale before it is actually been built or before the land has been subdivided.   As you cannot see the end product, the risks a considerations compared with other property purchases are much higher.
When you’re buying off the plan you enter into a contract with a developer on the basis that the developer would do its best to construct the building in a proper manner within the timeframe allowed in contract.
Always ensure you have a full written advice from your solicitor at eConveyancing NSW before you enter into any contract.