It looks like a Common Green Tree Frog (Litoia caerulea) that is a bit undernourished. If you want to help you must be prepared to “break” the law in the process. You can feed it up on crickets and woodies and mealworms and moths and other insects for two or three weeks... until it puts on some condition. Then let it go where you found it. Be discrete about what you are doing i.e. do not mention it to anyone as it is not legal by the letter of the law. The legal alternative is to find a wildlife carer in your area and to hand it over to them to look after and release. Your decision...
Wild frogs in general usually don't have weight troubles unless there is an underlying problem, I'd just let it go from whence it came and let nature sort itself out..... there may be a hungry Green Tree Snake around that can make use of a skinny sick frog...