I am thinking about buying an iPad Pro or MacBook Air (Skylake), as a first time buyer of Apple hardware.
I was wondering if I can use iTunes credit balance to help subsidise/buy this hardware in Apple Online store?
Unfortunately, no. One can only use iTunes credit to buy iTunes Store content and apps, not hardware. I think that it says that on the card itself in fine print, but I could be wrong.
I couldn't find an official statement from Apple, but here's something from the Apple support community: https://discussions.apple.com/thread/2592711?start=0&tstart=0