Welcome to the Bunnings Workshop community. It's marvellous to have you join us, and thank you for sharing your question about decommissioning your pool.
Before you contact anyone about filling in your swimming pool, I suggest checking out the rules and regulations of your local council. Some councils require that holes must be drilled into the bottom of the pool and its sides broken into small pieces. This is to prevent water from gathering in the old pool cavity. The local authorities in your area might have more requirements so it's best to double check.
I also recommend doing a google search on pool decommissioning specialists in your area and contacting them for a quote.