Part of my job includes payroll for about 300 people. I take nothing more than the prescribed amount for tax unless I have it in writing and can't imagine why you would. It is not up to me to decide whether an employee gets a tax return or not - that's up to the ATO! Personally I'd rather it in my bank account doing something than in theirs - I give them quite enough already.
doubt that it's illegal
it's not illegal to make voluntary contributions and it's probably stated somewhere in terms of employment that they do it, which makes it voluntary contribution
