First, the interesting question is not whether the cause of house-price inflation in New Zealand is supply, demand, or some combination of both. Obviously, since house prices are set by mutual agreement between buyers and sellers, prices are always and everywhere the result of both supply and demand. Rather the issue is, to the extent that house prices are inappropriate for some reason, whether the source of the inappropriateness is acting through supply or demand. Matt frames this by asking whether something is pushing demand for housing beyond what is "socially optimal" (or, by extension, restricting supply below what is socially optimal). Another way of saying this is to ask whether the policy response to high house prices would work by increasing supply (say changes to zoning or consent processes) or demand (say, changes in the tax treatment of housing).
The second clarification is that saying that supply and demand are not mutually exclusive is more than just saying that influences on both sides can contribute to the final effect. In the case of tax policies, the purpoted cause of house-price inflation only makes sense if there is an underlying problem with supply. To illustrate, consider Matt's statement
The key point against supply side issues will be fact that rental growth hasn't gotten as scary at any point -- if there are "too few" houses, then we should really see the cost of housing services/rent pick up.The idea here is that if house prices are going up faster than rent, then the opportunity cost of owning a rental property must be rising faster than the direct income derived from it, so the only motivation must be the expectation of capital gain. This is true, but the expectation of capital gain only makes sense if the underlying trend is for the demand for housing for non-investment purposes to grow faster than supply. That is, to explain house inflation today as driven by tax-favoured investment, we need to assume a problem with restrictions on supply in the future.
Furthermore, consider what we would observe if there were no favourable tax treatment for owner-occupied housing or income derived from capital gains, but still an expectation of demand growth outstripping supply growth in the future. As long as the capital gains tax rate were not set at 100%, it would still be the case that expectations of future house price inflation would drive inflation in house prices today, there would be a positive after-tax return from capital gains, and hence a slower rise in rents, exactly the observations that Matt suggests might imply the problem is not exclusively on the supply side.
The bottom line here is that the favourable-tax-treatment story simply implies that problems due to insufficient supply will bite a bit earlier than they otherwise would have done. If there is no problem with supply being unable to keep pace with underlying demand, the tax-treatment issue is irrelevant.