As Bernie says, GDA (technically GDA94) and WGS (technically WGS84) are close enough for bushwalking purposes. If you need accuracy down to the centimetre, you will need to deal with the difference, but accuracy of < 10m is rarely required for bushwalking. Current difference I think is around 0.5m, but it changes as Australia moves due to continental drift.
GDA94 and WGS84 are datums, which are a model of the earth – that is somewhat curved.
Maps are a translation of the curved earth into a flat surface. Most maps for bushwalking are UTM (Universal Transverse Mercator) projections using a particular datum, and use a Cartesian (XY) co-ordinate system. For Australia, the datum used for projection is now GDA94 (=WGS84), but used to be AGD66, which you'll still see on many maps. The combination of the projection and datum is known as the Map Grid of Australia (MGA), which is why you will often see co-ordinates given as MGA456234 or similar.
It's much more convenient to locate a point on a bushwalking map using a 6 figure grid reference, than it is to locate the same point using Lat/Lon. Try it if you don't believe me. The base unit for the grid reference is the metre, so it makes calculating distances much easier. Maps are also divided into 1km squares, along the grid lines, so points can easily be interpolated.
The main advantage of Lat/Lon is that it is universal (well, ok, it's global!), which is why internally, GPSs and similar store track points in Lat/Lon.
For info on converting grid references, see
http://ozultimate.com/canyoning/map_grids.htmThis also has some info on datums and map grids and the difference.