I have an unused re-entry visa in my passport, valid up until the expiry date of the non-imm visa. If I then subsequently extend the Non-imm visa for another year and leave the country before the expiry date on the re-entry can I re-enter after the expiry date, i.e during the valid time of the new extention ? Or do i need to get a new re-entry visa before I leave, valid until the end of the new extention ?

Your re-entry permit is tied to your current permission to stay, when that expire so does the re-entry permit ( check the expiry date on the re-entry) So when you get a new permission to stay you need to get a new re-entry permit.

You will need to get a new re-entry permit. Your old re-entry permit expires on the date your current extension of stay ends (it is not a visa).