I can’t for the life of me find any details about this offer on the Myer website, but right now LEGO appears to be 20% off with no obvious exclusions. I suspect that this offer will run until Christmas Eve but I don’t know for sure.
If you want to save a little more then you can get a further 10% off at eBay with the code CAU10. Your mileage with the code may vary as some people haven’t been able to get it working. I did test it myself and it worked though.