Thank you for your post. This is a very good question and you have come to the right place for more information. It all depends on what the overall lending to value ratio (LVR) will be and that your income can support the loan. The LVR is calculated by diving the loan amount by the property value so essentially it is how much your deposit or contribution is. If you have less than a 20% deposit and your LVR will be greater than 80% then you will need to ensure that all probationary periods are completed and the bank would prefer you were with the same employer for at least 12 months with at least 1 year remaining on your 457 visa. They may look at strong applications prior to this and each case is assessed on it's merits.
You do not pay a higher premium or rate as a 457 visa holder but dependant on your deposit, the number of lenders to seek finance from may be less than a low LVR loan.
If you have a larger deposit and the LVR is quite low then there really is no minimum period of length of stay and as mentioned each case is assessed on it's merits. Having set up this 457 visa mortgage policy (migrant policy) and working in the migration industry for the banks, I can evaluate all 457 visa home loan applications before submitting them for approval and let you know my thoughts. From there I can walk you through the whole process.
If you have any more questions please do not hesitate to contact myself