It does not sound like an easy PL claim.
I would speak to a Victorian PI lawyer to see if they are able to assist and make sure that you do it as soon as possible as there is a 3 year time limit to bring a claim in court. However, if the other contractors left the mud on the porch you cannot really expect the owner to go after every contractor and clean up after them every 20 minutes or so. There would, however, be an obligation on the parties working at the premises to keep walkways clear and free from slip and trip hazards, but then you would have to identify the contractors who are responsible for all the other materials and the deposition of the mud etc (which was probably a combined effort) by the sounds of it.
You should look into the WC issue further with a PI lawyer, as I am not familiar with the VIC WC system.
Yevgeni