Hi Tommaki,
As the theft offence is more than 10 years old it will be a spent conviction and therefore will not appear on the National Police Check obtained by your prospective employer. However if the employment application form asks whether you have any prior convictions, you are still required to answer honestly. It will depend how the question is worded as to whether or not you need to fess up about it from a legal perspective. But from a moral perspective its up to you.