Don't get caught doing this as its illegal in NSW and I am not sure in other states!
Sam I was born and bred in Wollongong , if I remember correctly it's illegal to look sideways

Remember that shell grit is to aid with digestion because chooks don't have teeth. If it's calcium they are lacking oyster shell or calcium carbonate is is what you need. Although if it's reduced quantity in eggs it's more likely to be inadequate protein, not calcium.