Welcome to the Bunnings Workshop community @Ellshell. It's wonderful that you've joined us, and many thanks for your question about weatherproofing your deck.
An option would be to extend the joists with purlins adjusted to a slight fall and fix iron or polycarbonate sheeting to them. You can then install guttering to divert the collected water into a stormwater outlet. I've put together an illustration below to explain better.
There are potentially easier or cheaper ways to do this, but I am unsure how they would stand the test of time.
Is the deck fully exposed to the weather, and how much water drips through to the lower level?
Thank you for that advise. I'm more looking for something to lay on the floor of the deck cause the rain comes in 2 different directions, would also help if I over watered my plants😊
How much of the deck did you want to cover? Perhaps you could place your plants on some rubber floor matting?
My only concern about placing something on the decking would be where the water will then be directed. Obviously, it needs to go somewhere, and that will be determined by whether the deck has any fall or slope.
Have you considered installing a water system on a timer that utilises drippers? That way, there won't be any excess water ending up on the deck.