How did the public express "distress" before the campaign ever hit the sides of buses?

Or have they equated offense-seeking religious groups with public opinion?

A bit of both... the campaign was in the news over here before it launched.
New Zealand has a kind of "don't ask, don't tell" attitude to religion. There's a sort of general feeling that somebody's religious beliefs are roughly equivalent to their sexual fantasies in terms of how mentionable they are. People make tasteless jokes about them, of course -- New Zealanders will kill for our right to make tasteless jokes -- but any discussion that is not a tasteless joke is in poor taste.
Yes, I thought very carefully about that last sentence.
So when minority religious groups burst onto the national stage, it makes the general New Zealand public uncomfortable, and that has happened several times in the last few years: the attempted hijack of the 2005 general election by the Exclusive Brethren; the spectacle of Brian Tamaki's black-shirted Destiny Church mob; the fight against the children's rights legislation (misleadingly labelled "anti-smacking" by its opponents, but alas, the name stuck)... I think that's where NZ Bus are coming from.