My strong recommendation is that you seek alternate employment which complies with the requirements of your visa. Whilst it appears that your employer is flouting employment laws in a number of ways, if you find yourself out of work it may still create visa issues for you and it is no defence to complying with your visa requirements that your employer has forced you to either resign your job or fired you.
Once you have found a new job, or before if you are not concerned about the above, you could consider bringing legal proceedings before the Fair Work Commission for any issues arising out of not being paid for certain work, including vehicle collection, and you can refer your concerns about the non payment of superannuation to the Australian Tax Office which should deal with such issues.
As to the employment contract if it was 'approved' by your agent then it may well be valid (provided it complies with other employment laws and particularly your award -whatever that may be). I suggest you get a copy of your contract, and find out your award, either by asking your employer, or if that seems imprudent, contacting the Fair Work Ombudsman. The Fair Work Ombudsman can assist you in finding out the award applicable to you and addressing other concerns you have regarding your employment situation, and they have considerable useful information on their website.
Links to the above agencies can be found here:
I trust the above assists.
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