mySAWater automatically logs out if left idle. Can I turn this setting off?

No.  This security feature is in place so that if you accidentally remain logged in on a compromised or public device your personal information is not at risk.  You will be automatically logged out if your computer is idle for 20 minutes.  To discuss this further, please call 1300 138 941 during business hours (Mon-Fri, 8:30am to 5:00pm) and our Customer Care Centre will assist you.