It takes me 15 minutes to walk (I'm mildly arthritic in the legs) from the farthest gate and be through passport control using Smartgate at Sydney Airport. Melbourne is slightly longer. The longest wait is going to be for your baggage, and that can vary. I've never ever had my baggage waiting for me by the time I've got through Smartgate, so just ignore all the negative comments you've heard about getting from the plane to Smartgate. By the way there are several Smartgate terminals along the way. I always use the first one I come to unless there's already a queue, and then head to the Smartgate "gates"

I'd say you'll be out of the Arrivals side in (well) under an hour from de-planing. The so-called "shopping mall" is a single duty free shop of about 50 metres in length

Then it's straight upstairs (there are elevators) to check in. It could take a few minutes to find which check-in aisle you'll be using. It's not clear if you're on a VA aircraft or a codeshare with NZ; both airlines use check-in aisles from H onwards, usually. There will be queues but it's a small aircraft so that shouldn't be too bad

Outwards passport control is another Smartgate; there is an outbound card to complete for each passenger as well and you're supposed to deposit that in the appropriate box after Smartgate (I never have)

The real "shopping mall masquerading as an airport" is after hand-held baggage x-ray. The quickest way to avoid that is to look to your right to find a large sign about processing purchases bought in Australia for GST (sales tax) reimbursement. Head straight for that sign as it's a short-cut to the departure gates you'll be using which will be a gate number of 50 or higher. I'd say that you should have plenty of time; if you're delayed by the time you get to check-in the airline staff will organise a priority pass to get you through outwards passport control (slightly) quicker. They may even walk you through themselves

Good luck