I was wondering if there is a way to convert heights that are logged from a normal bluetooth GPS to AHD heights.
You'll need a Geoid model and a program to use it. This page - http://www.ga.gov.au/geodesy/ausgeoid/ - seems to give you both of these.
I'd also check the GPS to see if it has a similar geoid model built in and can auto-correct. Often they can - but the model is usually very coarse because of size constraints.