Welcome to the Bunnings Workshop community @glenb. It's wonderful that you've joined us, and many thanks for your question on BBQ installation.
Could I please confirm, is the new BBQ 30mm wider than the current opening, and you're looking to move the vertical pieces out 15mm on each side? If not, could you draw a diagram or provide a little more information on what you'd like to achieve?
I'd certainly recommend you provide more clearance than what was there was previously. The timber on the bottom looks particularly scorched and likely wasn't far off a fire at some stage. Fibre cement sheeting would provide more heat resistance than timber, but it's also essential to have an air gap. It would be my advice to remove the entire combustible timber recess and replace it with one made from James Hardie 2400 x 1200 x 15mm 2.88m² HardiePanel Flooring.
We also have a range of BBQ modules for in-built units that you might be interested in incorporating.
I look forward to hearing more details on your project.