According to the rules in most states sewage may only legally dumped in open waters (most say via a macerator) or onshore.
To satisfy the regulations I believe a "Thunder Bucket" from BCF, one or two plastic clip lid 10 litre paint buckets as storage tanks and a potato masher type paint stirrer as a macerator will suffice.
I posed the question to a Queensland Ports and Marine surveyor some years ago and he stated that it was a system that would comply with the regulations.
Chemicals to aid decomposition and deodorise are optional.