What I'm saying is that your employer has obligations to you, even if you are sick/injured. he cannot do anything to effect you immigration status if you are now a resident without S49 conditions.
Taking advice from A CAB should clarify what ACC and sick leave payments you may be entitled to, and if you feel you are being bullied into resigning by your employer then you could take action to stop that.
if you want to resigne, the notice period should be in your employment contract, or else the same period of notice is required, as you are paid. (weekly, fortnighly, monthly?)