An employee’s right to work in Australia needs to be thoroughly verified before you make an offer of employment. Don’t assume that everyone you employ is legally entitled to work here. Make it part of your on-boarding process that all new hires have to produce their passport or some other proof that clearly shows their right to work in Australia. Sight original documents only and take copies. If in any doubt at all seek help from a migration lawyer, especially if a 457 Visa is involved.  The fines for employing someone who is not eligible to work in Australia are huge.