Need help mounting a TV to the wall
If the top of the plug is shearing off when you put the bolt in, then this means the bottom of the plug isn't turning (so not a problem of the hole being too large), but the top of the plug is turning (which means the bolt is gripping it rather than turning within it).

Looking at the picture of your bolt in the partially failed plug, and given the above, I believe your plug diameter is too small for your bolt. Your options would therefore be to either get smaller bolts, or to get larger plugs (and drill a larger hole for them).



An alternative would be to get a different sort of plug. There is a product called "grip-it" designed to attach to plasterboard (=drywall) which has been installed dot-and-dab over masonry as per your situation. I've found it to work well, but the diameter of the fixing is very large, so is only suitable when the bolt won't be near to a visible edge. The business started in the UK, I have no idea whether it's available in Australia.