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Afternoon @AussiePossum
HArd call determining what animal it could be that has died. I am not sure if the baking soda would work as its an open room (fridge it works brilliantly) I would probarlly go with the peppermint oil for now as it seems to be working.
As for new mice moving in, I would have a look around, if you can look under your house great, if its a slab then for stuff up against the outside of the house. I would suggets some traps for mice would be a better way to start with then an exterminator. Cheaper at the very least and you get to choose the type of mouse traps you want. Mouse Traps, Rodent Controls & Repellents for a range of them 
Dave
Hello @AussiePossum
It's great that you've received excellent advice from @Dave-1. It definitely sounds like you’ve got a dead mouse in the wall, especially since you heard scratching about a week ago. The smell typically peaks over a few days and gradually fades as the carcass dries out, but it’s understandable that you’d want to manage the odour and prevent future problems. Placing a small bowl of baking soda near the source is a good idea it won’t eliminate the smell completely, but it can help absorb some of the odour instead of just masking it like peppermint oil does. It’s not a miracle fix, but it’s worth trying. If you want something more effective, Bunnings sells products like DampRid Odour Genie which can do a better job of neutralising odours.
As for prevention, it’s absolutely worthwhile to have someone inspect the exterior of your home for entry points. Even in a new, well-sealed house, mice can squeeze through gaps as small as 6mm—about the diameter of a pencil. Common problem areas include weep holes in brickwork, gaps around utility pipes and cables, roof eaves, vents, and under doors. A professional pest control specialist or experienced handyman can help you identify these areas and recommend sealing options, such as wire mesh, steel wool, expanding foam, or proper caulking to block access.
Using peppermint oil spray outdoors can also help as a deterrent, especially around potential entry points. It won’t guarantee that mice stay away, but it can discourage them from approaching. Bunnings typically stocks RATSAK Fast Action Wax Blocks - 66 Pack which is a fast action rat and mouse wax blocks kill rats and mice with a single feed. Easy to use, they are mould and wet weather-resistant, so they can be used in wet and dry areas. Alternatively, you can make your own spray by mixing water, a bit of dish soap, and peppermint oil in a spray bottle, then applying it around doors, vents, weep holes, and the base of your exterior walls.
As a broader strategy, take a look around your yard for things that might attract rodents, such as bird feeders, compost bins, or dense vegetation near the house. Combining regular exterior inspections with sealing and deterrents is usually the most effective way to keep mice out long-term.
If you need further assistance, please let us know.
Eric
