I've come to a "solution". At least on Mac, you can use chrome-cli to control chrome, so now I keep an NZ Post tap open forever, and the script runs every few hours and will log me in.

Is it safe/secure?... eh. Does it work? You better believe it does! No more NZ Post logging me out.

I do keep my password in my keychain, so at least it's not as plaintext in the shell script, but how safe it is after that idk.

Here is the script if anyone is curious, use at your own risk.

https://pastecode.io/s/rar57pys

The biggest issue thus far, is it'll push chrome as the active window when the script runs... But it won't force you onto the tab, which is something.

Update: Nevermind, turns out `chrome-cli open` command is what forces chrome to get the focus. So changed them use JS to change the url. 

Today is a good day.