First - I'd recommend backing up your /etc/apt/sources.list file first:

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

Then run sed across it to change it to the AU mirrors:

sudo sed -i -e 's/nz/au/g' /etc/apt/sources.list && sudo apt update

To get it back to the NZ mirrors just run it in reverse like so:

sudo sed -i -e 's/au/nz/g' /etc/apt/sources.list && sudo apt update