Low balance notification is an alert for when your account balance drops below a specified level.
This is most often used so that you don't run out of credit on a prepaid account, for example if your monthly charges are $100 the system can notify you once your account is below $105 to make sure you always have credit for your monthly charges and some calls.
This feature can be changed by loging into the customer portal at: https://customer.voipline.net.au/
Here you can enable/disable the service, set the notification threshold and what email the alerts are sent to.
Once logged in the settings area is at the bottom right of the initial login screen as highlighted in the red box below.