For the wireless experts out there, what is the viability of broadband over wireless if Spark utilise more spectrum and maybe offer an antenna on the roof to take advantage of the higher frequencies?

Not viable going forward.

I am not aware of the total spectrum holdings spark has, nor the capability of LTE but I would think they would be lucky to have 500mbits of airborne bandwidth in a typical suburb, spread amongst 1000+ households.

Compared to UFB which can fit 100mbits down a single strand of fiber to every house.

Radio is also a finite resource - its running out, with the only way to increase capacity is to find ways to use it more efficiently, or make coverage cells smaller (with fiber backhauls to the new towers) so the transmission capacity of each tower is shared amongst less houses.

However fiber to the home is, in this example, an unlimited resource with 1gbit being able to be delivered to each household, with 10gbit already available but requiring a hardware cost drop which will come in time to bring 10gbits to the mainstream. 

So if you were to think about the number of megabits being able to be pumped into a suburb, fiber to the home can deliver more megabits per household than point-to-multipoint cellular radio.
And with netflix etc requiring 2 to 5 megabits per household from 6pm to 10pm all at once for tv watching, fiber is the only one that can deliver going forward. Radio will always lag behind urban density requirements.

Cellular radio can deliver to poorer suburbs where a larger number of subscribers would rather not have a fixed line connection, and would prefer the tradeoff of less data for a cheaper alternative. Still in these suburbs I see fixed line fiber being the preferred choice as its becoming more and more viable to drop the sky subscription in favor of netflix + broadband.

True, the other option is a flat filled with students who don't want to be lumped with bill after half their flat mates bugger off at the end of the year. That being said, I had a look at the availability and it appears that it is mainly focused in the low income areas such as Cannon's Creek and Ranui.