Hi @handiher19,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you join us.
I'd suggest ensuring you have a good strong floor with no weak points before applying the sheet material over the top. This would mean filling these holes.
As it looks like you've got good access to the underside of the house, I'd suggest attaching some 12mm Plywood Hardwood Marine AA Grade to the underside of the floor by screwing down through the existing floorboards using 10G x 30mm Timber Screws. You will need a second set of hands to help you with this.
Please ensure you communicate to avoid any stray screws ending up in somebodies' hand, speaking from experience, it's not a fun time. You could use a piece of scrap timber to hold the plywood in place, so your hands are well clear of the screws.
You can then use Builders Bog to fill the void up to the level of the hardwood floor. This will ensure there is a nice flat surface with no voids that you can lay sheeting over.
You can then use Ceramic Tile Underlay Flooring over the existing hardwood floors and a product such as Villaboard for the walls. These are both fibre cement products that are the standard material used in wet areas such as this.
With these products, ensure you follow the manufacturers installation instructions (Hardie Ceramic Tile Underlay Installation Guide and Villaboard Lining Installation Guide) to ensure a strong, safe and warrantable installation.
Let me know if you have any further questions.
Jacob
