The interlocked factors (I ranted aboot this yesterday here) are just too hard to tackle, even individually.
One alternative (which may happen anyway) is letting the whole schemozzle run into the inevitable immovable object. Tempting as that is to those with apocalyptic/Malthusian tendencies, it would cause a lotta grief to a lotta bystanders. Collaterla damage.
I see (feeling more optimistic today, y'see) four aspects which can, together, make a bit of a breakthrough.
1 - wipe the zoning nonsense. MUL's etc cause price differentials, which firstly crystallize as CG to the lucky owners, then propagate to all and sundry (who, after all, would sell their hoose for an agricultural land price when your despised but now cashed-up neighbour has just made a cool $300K cf his original purchase price?). Luvverly free CG all around - after all - we didn't buy the house to Sell it! - but of course paid for by you-know-who. Guibmint by fiat could accomplish this by lunchtime tomorrow if it put its mind to it. What Constitution?
2 - Factory builds. Proper QC, can be erected in hours not months on site, will standardise foundations etc (one of the beefs in Chch is the continuing dither over TC3 founds, which need individual geotech to establish), and being built to tight tolerances and under cover, are a generally superior product to yer average chippie-hammered together set of frames (one of which, just down my street, has been sitting out in the weather for six weeks now with no roof and no wrap - nice.)
3 - GST exemption to factory builds from accredited suppliers. This avoids the obvious problem with a blanket 'GST exemption for new builds' which will be an easy route to no-GST decks, pergolas, baches, new rooms and re-roofs - because as GST has to be taken on raw materials, who can police what they are used for? No such issue with factory builds, which will have a BOM, a nationally valid type certificate of compliance, and a serial number.
4 - The cultural attitudes and legal provisions which make such builds acceptable to buyers (who, let's face it, have wildly inflated expectations of first homes, fed by TV house pron, and egged on by the usual bevy of marketeers), and to developers who are rather fond of placing CCR's (covenants, constraints and restrictions) on their conditions of sale. If we are serious aboot FHB's, then fer crying in the sink, let them set up next to You.