Hi Philip, The tropospheric Ducting is an atmospheric
phenomenon that just comes and goes, depending on weather patterns around the world. If you have a look at the
DX Info Centre site for the current forecast, you will see that there is
Good to
Strong ducting between Australia and New Zealand at the moment (you can select the region of the world at the top of the page)..
From memory, one cause can be hot inland air moving out over the colder ocean, and temperature inversion plays a role as well. I have seen large areas of ducting extending way out to the northeast of Sydney, and east of Brisbane, because of hot air moving out to sea from inland Queensland.
However, the ducting forecast is just like a weather forecast, where nothing is for certain, but it is usually a good guide.