My girlfriend is in Australia. She's an NZ citizen living in Australia. I'm from Pakistan and we want to marry under Immigration Law. Tell me process, thanks.
Best thing would be to find a celebrant in Australia (a simple Google search for your area would turn up something), tell them your situation, and have them walk you and your fiancé through the process.
Try using the contact details on the Find a Marriage Celebrant page of the Australian Attorney-General's website to find a celebrant near you and start from there. Also see their Getting Married page.