I found it difficult to find a Marine LPG gas plumber but then discovered
a WA Government site that lists all tradesmen with their trade certificates including Gas plumbers, electricians etc. Sorry but I'm not allowed to give you the link if you put "http" in front of this "://www.commerce.wa.gov.au/building-and-energy/find-gas-fitter-electrician-or-plumber" it may work
When I contacted a gas fitter on line he decline to quote as he said he only worked in QLD and wasn't prepared to travel to WA!. But he offered some very good advice. "Why don't you ask your local boat broker to recommend a plumber"? I did that and I immediately had two tradesmen who have agreed to quote.
From now on that's the approach I will take whenever I need a tradesman.