What metrics would indicate a house bubble rather than genuine market values? There are concerns that Auckland, New Zealand, is currently experiencing a housing bubble. 

Auckland is one of the top 10 cities in the world on a housing unaffordability index. 

The question is - how as economists can we tell whether housing prices reflect a bubble or genuine market value? 

I would be looking at average wages, crime rates, happiness, and other social measures of the city (ie. it's reasonable that people would be willing to spend more on a house in a utopian city). A good answer should comprehensively list these and explain the relevance of these metrics.
The first metric to look at is house-price-to-rent ratios. Rental prices capture the value of the housing (and housing-linked) services provided by a property, including things like how safe a neighborhood is, how good the schools are, et cetera. In contrast, house prices are the capitalized value of the future stream of housing services PLUS the price of an asset (mostly land; structure value beyond the housing services it provides is negligible). As a result, a higher ratio of house prices to rents means that, controlling for the value of housing services provided, we observe people paying more for housing assets (i.e., land).

This doesn't mean that the asset is necessarily overvalued, but it at least moves you in the right direction by separating out the asset price from the value of housing services. You can then start thinking about the supply/demand fundamentals that apply to housing assets, and what you'd have to assume to support a particular valuation of the asset (things like, as Jamzy noted, whether growth in price-to-rent ratios is linked with increased home purchase borrowing).