Unable to install mysql-workbench for Ubuntu 16.04 LTS

It seems that you forgot to run sudo apt-get update before installing MySQL Workbench (Australian mirror is synced normally, it has correct version of gcc-5-base in pool).

If it does not help, try to switch to Main server (Software & Updates or software-properties-gtk, Ubuntu Software tab, set Download from to Main server).

For sure add all official repositories with:

sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository multiverse

Make sure you have enabled xenial-updates and xenial-security sections:

1. open software-properties-gtk
2. go to Updates tab
3. check first two checkboxes - (xenial-updates) and (xenial-security)
4. enter password, click Authenticate
5. click Close, then Reload and wait it to finish.

Then run

sudo apt-get update   
sudo apt-get install -f
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo apt-get install mysql-workbench