Hello,
Just wondering is there a way to display a message instead of the Add to Cart button if the product weight or product size is over the shipping method recommendations?
There is no option to do that, available shipping methods are defined during the checkout workflow, so you'll have to write an algorithm on your own regarding your shipping methods so that you can know if there will be an available shipping method for your product page.
You can add your custom code via the "Show_default" file of the "Product" view of your front-end template.
" Australia Post has a weight limit of 20 kg and a volume limit of 0.18 m3"
In your case you'll have to check your products weight/dimensions, and if it exceed the limit, you'll have to display your error message.