Not too hard if it's an OEM item, Mania & a few other places may have a 2nd hand back. If it's after market like a Robins, you may need to contact an after market crew (don't know of any aussie outlets off hand for the Robins).
Or you could just remove the backing (
check out the miata garage) and take it to an upholstery place to sew a new one in, but it costs about as much as a new backing anyway.
I'm actually planning to do a DIY on mine this weekend. Mines all faded and I've tried all sorts of stuff to fix it, even silvo/brasso. So going to unstitch mine and break out the overlocker & some silicone sealer.
Figured I'd give it a go for fun before buying an entire new top after I get my tax out of the way in a few weeks.
CS