My Freeview reception suddenly became troublesome last Friday on all major channels except TV3. A few days later only Maori and Te Reo were affected. A re-tune simply lost them altogether but my home theatre PC's tuners could pick them up.  The TV reported that channel 39 signal as weak with zero quality.

Long story short, it turns out the antenna I purchased from Harvey Norman 2 years ago did not like the vertical polarisation required for Hawkes Bay's Mt Erin. The plastic electronics enclosure was designed for the cable to exit downwards with the vents facing down, as oriented for horizontal polarisation.

Turned 90°to suit Mt Erin the vents were on the side and the box filled partially with water during a thunderstorm on the prior Thursday.

I can only imagine that horizontal is the more common scenario and so this situation was not recognised.