Per the Australia New Zealand Food Standards Code - Standard 3.2.3 - Food Premises and Equipment:
4 Water supply
(1) Food premises must have an adequate supply of water if water is to be used at the food premises for any of the activities conducted on the food premises.
(2) Subject to subclause (3), a food business must use potable water for all activities that use water that are conducted on the food premises. (This would include hand washing)
(3) If a food business demonstrates that the use of non-potable water for a purpose will not adversely affect the safety of the food handled by the food business, the food business may use non-potable water for that purpose.
You can rely on your water supplier to provide you with their micro water analysis data - which should be available online from their web site.
However as the water supplier has no control of the water quality once it leaves their facility and enters your site, how do you know there is not a source of contamination occurring in the water on your site? How do you verify that the water being used meets the requirement for being potable?
Do you have taps/washbasins hoses etc that are not used for long periods of time? Dead ended water lines? Only being used for handwashing, are you certain? What about for drinking, what do your operators do there? There is always the possibility for the water to be used on site by your employees for purposes you are unaware of, assumptions can be a very dangerous thing.
I would be randomly testing the water quality ex the potable water outlets over months, testing for coliforms and e.coli per 100ml (A requirement for dairy in Aust.)
Collect some data to show there is or is no issue with the water quality, handwashing only or not. Once you have the data you can review the risk and change the frequency of testing. Without this data you're only working to assumptions and are flying blind.