For most people, it would seem that around 12" or more of aperture is required to pick up faint colour in M42.
If M42 is a "faint dust cloud" through your scope, you must be suffering from much light pollution. Try to get to somewhere 100km or more from the outskirts of Sydney and try again.
The DGM NPB filter gets good reviews for nebula
http://www.omegafiltersebuyer.com/se...ers/Categories
I have one and it is great on the Eta Carina nebula. Others say an OIII filter is good as well. But I only use it where there is no great light pollution. Perhaps something else would be better under your sky conditions.