Hi there,

I'm in Auckland and most pet shops with a good reptile area have dusting powders. Most of them are marketed towards reptiles but as you can read on packaging are also for amphibians.
Fluker's is a well known one. They do a calcium powder with or without vitamin D3 - depends on your animals UV exposure and requirements. They also do a multi-vitamin spray and probably more I have not encountered. I have seen these products at Animates stores, and also at Bird Barn in Henderson. I got the calcium + D3 powder for my green and golden bell frogs - have only used it a couple times when I catch moths - they are easy to dust compared with flies!

My understanding is that Whistling tree frogs are nocturnal so if you were to get a calcium powder go for the one without vitamin D3 as it is unlikely they require the extra D3 for calcium absorption - check on that though!
Remember too much can be just as bad, if not worse, than not enough when it comes to vitamins and minerals (apparently especially for D3).
If you are feeding your frogs a balanced and varied diet they probably wont need supplements, but are handy to have around when variety of food is scarce.

Hope that helps