Mike,
Are all VHF TV broadcasts in NZ horizontally polarized, or just the ones from your local transmitter?
When VHF was used for TV in the U.K. we had a mix of vertically and horizontally polarized transmitters (I have some links to websites with the old assignments somewhere -- I'll try to find them later).
Combined band I (low VHF) and band III (high VHF) antennas were available, but separate aerials and a diplexer were more common in many areas, as the BBC and ITA transmitter sites were often in different directions.
When the UHF network was planned in the 1960s, they came to co-siting agreements, which meant that only a single UHF antenna would be needed for all stations. (That planning has been totally screwed up in recent years, but that's another story.
)
The UHF network here uses horizontal polarization for the main transmitters, with vertical for
most of the relays (low-power transposers).
People in parts of Wales and Scotland (the mountainous regions of Britain) were not happym when VHF services were closed down in 1985, as it left some areas with very poor signals from the UHF broadcasts.
For some odd reason my 2 metre Transciever knocks out some of the UHF Channels
Could be harmonics, although you'd be looking at the 4th harmonic and above, which should be pretty well suppressed. Many of the UHF masthead preamps for TV use tend to be easily de-sensitized by strong fields nearby, or you may be getting cross-modulation products.