That will depend on where it is. In the smaller sections in Stage 2 the site coverage is currently 40%. This means that for a 400 square metre section you cannot exceed 160 square metres of build floor area. You may however exceed this limit with subject to Council consent and this is a formal application. A number of owners have been successful in achieving this, however there is a limit to how far you can push it.
In all other sections including the whole of Stage 3 the maximum site coverage is currently 35%. The Council is often more lenient with variations where there is open space around the property such as a green space and where there is negligible effect on neighbours.