As above, altitude accuracy is usually 1.5 to 2 times worse than horizontal accuracy - unless you have locked on to 3 or 4 satellites near the horizon, in which case it's likely to be as good.
If you do as woody suggests, and use the Aussie datum, then your horizontal position would be better, but for accurate vertical position reporting you need to get your gps to give you height in terms of the Australian Height Datum (AHD71). That's what is used for the values printed on maps, and is based on mean sea level around the continent, rather than distance from the earth's centre of mass (which is the reference point for all gps positioning). Naturally, the different reference levels give differing heights.
:)
