Small 12v Battery usually used for garage remotes or other types of remotes are on sale.
The are also known as 27A, GP27A, MN27, L828,G27A,V27A, and G27A
They usually go for about $5 each and probably $20+ in the malls.
It can be pricematched (to save 15 cents on Mitre 10)
https://www.mitre10.co.nz/shop/energizer-alkaline-battery-a2…
Might be just your local OP and in response to the price at a local PNS store
https://www.cheapies.nz/comment/224854/redir
Price is $4.90 at Bunnings when I click your deal link.