Mike,
I'm not sure how you protect Ashburton area but in Auckland OHUG lines have surge arrestors fitted at the crusifix and at both sides of the new gas switches which are replacing the ABS switches.
The LA's are their to protect the cables and gas switches against excessive voltage surges.
Check all lightning arrestors on your outgoing lines which drop ddo's.
We have had the odd intermittand ones taken out a feeder on earthfault or sometimes overcurrent in Auckland.
Usually the LA's blow apart but sometimes they don't, and look ok from ground level, but require a close look and a bulged end or tear in the resin may be visible.
When temperatures change, the tracking across it may increase and cause a blow out.
For 11 kV we use LA's with MCOV value of 8.4 kV. which is adequate for the RMS value of 6350 Volts to earth.
Somehow we got also 7.65 kV MCOV LA's which tend to be more prone to spikes or lightning strikes.(perhaps these were cheaper)
It is not likely to be a cable fault because that will usually drop ddo's out immidiately after re energising, or within a couple of minutes.
A faulty insulator is also possible and then feedback from a local resident can be very helpfull re a bang or flash noticed along a line.
Good luck with fault finding.
