Thanks so much for the advice
When I tried to pick it up off the drain grid I realised that its throat was stuck in the drain grid and that's why it wasn't moving and had that hunched over look. Something very hard (insect?) was lodged in its throat which was well and truly stuck in the drain grid. I was scared of perforating its throat if I pushed hard on whatever was in its throat causing it to be jammed; so I took the tree frog on the drain grid in a bucket of water (frog was only partly immersed) and headed off to Currumbin Wildlife Hospital. The vet was able to free the frog within minutes (didn't see how this was done) and medication was given to the frog (part of of his throat looked very sore - he had clearly being trying to tug himself free). Do hope that he recovers and that he can come home again.