Thanks for the tips! I'll see what I can find and let you know how I go.If anythings for sure don't do the method that I described in the quoted post it ended up being a waste of time. Vacuum bagging is really the way to go as it gives you very smooth parts needing minimal extra work to finish as well as giving you very good adhesion between layers.
I make all my molds out of Ultracal. I'm not sure if you can get it in Australia without incurring some ridiculous shipping fees but if you want to try using cornice cement I would test it on a small scale for minimal material waste. Building type cements tend to be a poor choice as they're usually meant to plug gaps and not adhere to fine details of a given surface as mold making materials are, and they also tend to be more porous, which will affect the quality of the finished part significantly. You can also use plaster of Paris which takes up detail better but the molds tend to be very fragile and generally don't survive making more than one or two sets of parts.
I used to use dedicated release compound supplied by Sig for molds but upon running out of that and discovering that another tiny container of it cost like $10, I tried carnauba wax (the stuff you polish car bodies with) and it works just as well and can be bought locally pretty much anywhere.