Home water tank treatment/UV filters etc is only a fairly recent development in NZ, it's only been trendy as townies move to tank water only areas.
Most people I know on tank water don't have any filtration/treatment and I've never heard any reports of mass sickness or higher than average water borne disease amongst them.
We have a single stage mechanical filter which normally has a 1um or 5um (depending on which store I happen to be able to get to when I buy a new one) cartridge in it. We used to use the carbon ones but they restrict water flow to as low as 33L/min which makes showers unimpressive compared to the nuclear decontamination flow rate we would get otherwise.
Anyway ... I do not believe we have been noticeably sicker from any bugs than we were on town water. But chlorinated water now tastes revolting.