Rely on the professional inspection you’ve already had done. Having it done correctly now is a million times better than relying on a future warranty claim. You don’t put worn tyres on your car just because you have insurance for a future crash.

This article may be helpful. https://www.bayset.com.au/Bond_Breakers_in_Waterproofing_Systems.html

"Builder says as long as shower area is waterproofed correctly this is not an issue as their warranty covers any waterproofing issues in future."

Which warranty exactly? Do you have anything in writing?

A 'warranty' sounds useful but you need to read the fine print and know what they cover and what exemptions benefit the builder.

Then there is what is said and what is done. After you have made your last payment getting a builder to come back to rectify something is a lot harder. They might avoid saying outright that they won't fix it but they can never find the time to do it. Then you're into talking to lawyers territory.

Then there's the reluctance to spend their time and money redoing anything they overlooked or took shortcuts with. It's much easier for them to tell you that you don't need something than to redo it properly. It will probably be alright for a while and by the time you find it isn't, see previous paragraph.