hi paul
wifi uses an unlicenced band so there are no people to approach to see if its legal. the only thing you need to adhere to is the power ouput level restriction (i think its 4watts or 36dB - i could be wrong - there is a .govt site that tells you here
http://www.med.govt.nz/rsm/licensing...ormation.html). a nice thing to do is a site survey for other wifi users to make sure you don't cause interfence on channels used by others.
for getting use of tall buildings to put gear on you may want to put a proposal together for the owners outling what it is you plan to do showing benefits to the community, how it may effect them, what it looks like etc etc and call them sir or maam not punter.
hope this helps
lata
mos