While I do agree with measuring the pH before really getting serious about adjusting it properly, I don't think the water reports are as rough of a guide as they might seem.
The reason the ranges appear to be broad for Brisbane water is rainfall in the dam catchment areas. There is limestone everywhere out there and when it rains substantially on it, it ends up in the water supply and causes it to go temporarily harder. If there's no decent rain for a long time like this summer has been like then the water profile stays pretty stable. You might have 20 weeks in a row where it all reads pretty much the same and then a random spike; it doesn't mean the water profile is all over the place though.
With the rain last week the water might be a little bit harder for a short while, but then it will return to its normal state. Meddo posted a chart last year of water profile testing that contained weekly analysis reports over a 3 month period or more. Most of them were pretty much the same readings, with a random week where the numbers jumped. I suspect there was rain out west shortly before that week's analysis was done.
My dad was an industrial chemist back in the late 60s/early 70s, and they used to test the tap water regularly in the lab... exactly the same thing happened after decent rain over the catchments.