@PollyWally ....hmmmm, yep I'd trot down to Spotlight and have a look at their mattress protectors and also their batting. The poly will be cheaper than the cotton or bamboos. I wonder that the cotton and the bamboo, especially the Spotlight stuff may be a bit too thin for your purpose and liking. I've also heard that some bamboo shrinks.
You know what I would do, living in Australia too...I would contact Nu-Wool, or Matilda's Own...both Australian companies and ask them which of their battings would be suitable for your play mat. Also/or contact Australian online suppliers like OzQuilts...they're terrific people to deal with.
Don't disregard the local quilt shops...often their batting is on a par cost wise to Spotlight.