Replace default apt us mirror (http://us.archive.ubuntu.com/ubuntu/) to au mirror (http://au.archive.ubuntu.com/ubuntu/) in Vim:
:%s/us.archive.ubuntu.com/au.archive.ubuntu.com/g
Replace default apt us mirror (http://us.archive.ubuntu.com/ubuntu/) to au mirror (http://au.archive.ubuntu.com/ubuntu/) in Terminal:
# For MAC:
sed -i '' -e 's/us.archive.ubuntu.com/au.archive.ubuntu.com/g' /etc/apt/sources.list
# Other Linux: