Welcome to the Bunnings Workshop community. It's wonderful to have you join us, and thank you for sharing your question about what wood to use on your ramp.
One of the main features of outdoor timber is its ability to withstand the weather and insect attack. Usually for a flat ramp I would recommend Marine Plywood as this is treated and made for outdoor use. However, you mentioned that the wood currently attached has grooves in it. We're going to need to see it so that we can get a better idea of what it looks like.
Once you post the photos, it will give our members a chance to assess the ramp. We can then make recommendations on what wood to use.