Believe it or not, that was one of the search phrases that a local Sydneysider googled. I know I actually wrote two posts on how to hide assets which you can read here, and here but I had no intentions to assist with deviousness of hiding assets from your marital spouse.
Obviously exceptions exist, such as your spouse is a gambler or a junkie or pretty much a useless good for nothing but generally, I'm not particularly interested in the exceptions to the rule when I'm writing for this blog.
If someone feels desperate, worried or concerned enough to try and hide money from their girlfriend or de facto then maybe they ought to reconsider the relationship. The best way is probably just being honest and straightforward. If necessary, get a BFS drawn up if you're really concerned about your assets.
If the relationship fails at the BFS contemplation and phrasing stage then that should solve your concerns. This is probably why people don't really stray too far from their social circle and from their peer groups. When entering into a relationship or seriously thinking about taking it one step further, money usually only becomes an issue when one party has significantly more than the other.