I live in the country and normally drink rainwater. When I visit some cities including Adelaide and Canberra, Stanthorpe in Queensland and Mansfield in Victoria, within an hour or so of drinking town supply water I become ill. Symptoms are feeling nauseous, increased heart rate and lethargy.
I suspect it is the chemicals used to clarify the water. Adelaide water produces the most severe symptoms. Any clues?

Short of going to an allergy doctor for testing it would be hard to tell.

I do have to laugh since normally it works the other way around, people from the city can get ill from well or fresh water sourses

Is the city water fluoridated? Some people claim there are those with allergies to fluoride and list a plethora of symptoms and side-effects. The scientific community at large discredits this - but who knows? Search the internet for "fluoride allergy" and you'll get more results than you'll ever be able to read... Next time try drinking bottled water (natural spring) and see if you get the same symptoms. In the US, waterworks are required to test water regularly for a variety of agents and the results are available to the public. See if you can get water testing results from the various cities you frequent and look for what stands out. Compare Adelaide carefully with the others since it seems to be the worst.

Where I come no one drinks tap water...
In fact we always buy packs of mineral water!
Better safe then sorry!

Boy oh boy! I wish I could help! I'm looking for answers as well! I used to drink water like it was going out of style! Then - within the past year - I become so ill from ANY water I drink! Be it tap, spring, mineral, bottled, filtered, hot/warm/cold - ANY water! And I haven't been able to get any answers from any doctors!! If you can find out what's going on - please - fill the rest of us in!!

Since 1994, in my mid-fifties, I have been allergic to tap water that is not treated with reverse osmosis. I lived in Hawaii at the time, but found no matter what state I went to, I would, within 10 minutes of ingesting water, have severe cramping and the runs. I am now able to use the reverse osmosis water piped into some grocery stores. I have to be very careful what I eat at restaurants is not made with water, including reconstituted juice. I hate having to quiz the servers. When I eat at close friends or relatives homes, they, thankfully, use reverse osmosis water to cook with on my behalf. I can use distilled water, but understand it is not good for you on a regular basis. I wish I had some answers, too. At least it is good to know we aren't the only one with this problem.