Your GPS tells you your ellipsoid WGS84 altitude.
"GPS does not report the height above the mean sea level, rather the GPS system compares the height to the WGS84 reference ellipsoid which may be above or below the actual sea level. In different parts of the earth it can be off by more than 200 meters (depending on the mass distribution of Earth). For example the geoid's surface around Florida is above the mean sea level by a good 30-40 meters, which means that standing on the shore would show you -30m as altitude. This is normal, and not an error, and caused by the fact that the altitude is relative to an artificial reference surface and not to the sea level. If you are interested in this topic, I recommend to read this
http://www.esri.com/news/arcuser/0703/geoid1of3.html
To find your true altitude above sea level go to this website.
http://www.ga.gov.au/geodesy/ausgeoid/nvalcomp.jsp
N is the difference between your sea level altitude and the WGS84 altitude. (take N off the GPS altitude in NE Australia)
In Brisbane take 41.5m off. In Perth add 32.5m.
This map shows N values
http://www.esri.com/news/arcuser/070.../geoid3_lg.jpg
If you have an Android phone this GPS status program is handy
http://eclipsim.com/gpsstatus/