Not sure the local rocks would have added to your problems. Canterbury rocks are pretty safe. All some rocks do is alter the pH, making it too alkaline. Limestone does this for instance.
To test a rock to see if it is safe, pour a little vinegar on it. If it fizzes, that indicates lime present and don't use it - unless you are setting up an African tank of course and want a high pH. I am sure though that the rocks had nothing to do with your fishes' dying, just the problem of too many fish too fast too soon.