My golden comet has had chalky shells three times since I got her. Each time was a day after she had some major stress. The first time she got sopping wet, and it seemed I couldn't even get her dry with a towel, the second time the coop got to 110degreesF, and the last time she collapsed from heat prostration. The day after each of these events she laid a chalk shelled egg. Inside on each time the egg was normal.
If you are sure that your layers with chalky shells don't have stressors, then I would look at their nutrition and water. First and easiest would be to put 1-tablespoon of apple cider vinegar per gallon in their water. Then I would make sure that they have enough feed, vitamins, minerals and available calcium sources such as oyster shell and crushed egg shells.
I know that it is winter there in Australia, so I doubt that you would be fighting the heat like we are in the states...
Hope this will give you some ideas....and hope that the measurements will be something you can convert to litres etc. Don't expect changes overnight --- and give them time and I bet that your hens eggshells will be fine.