Airspace in northern parts of the UK is closed with all flights cancelled, while airports in Germany have also shut following the spread of the ash cloud from Iceland's Grimsvötn volcano eruption.
By 0600 GMT on Wednesday (1600 Sydney time), the Volcanic Ash Advisory Centre forecasts that the plume will cover most of the UK, with an exception for the southwest, the Midlands, Wales and Northern Ireland.
Why is this a problem? Check out our article with maps showing air routes that cross the path of the cloud.
Airports in northern German cities will close from 0500 local time (1400 Sydney time), although main hubs Frankfurt and Munich are expected to continue operating.
UK airports Londonderry, Glasgow, Edinburgh, Prestwick, Durham Tees Valley, Newcastle and Carlisle are shut too.
Here's the forecast map for 0600 GMT Wednesday (1600 in Sydney):
The Netherlands (including Amsterdam's Schipol airport) will also be covered by the cloud, as will all of Denmark, half of Belgium, part of southern Finland, most of southern Sweden, a good part of Norway, northern Germany, northwest Poland, and the western half of Latvia, Lithuania and Estonia.
Crucially, the polar jetstream winds are also carrying the ash cloud as far west as Canada, across Greenland and into central Russia nearly as far south as Kazakhstan, and a band of the ash stretches south into the five main air tracks from the eastern US to the UK.
By 1200 GMT on Wednesday (2200 Sydney time) the cloud is forecast to move south and east. The skies above the UK are expected to be clear, although the approaches into London airports from anywhere but Spain, Africa and South America are still in the aviation ash "red zone".
That means that flights from Australia to Europe are likely to be affected.
Passengers should expect delays as aircraft fly around the ash cloud, and potential diversions if the planes haven't loaded enough fuel to be able to skirt around the edges of the volcanic plume.
Do you need to reach Scotland or northern England while airspace is still closed? Check out our guide to your rail travel options.
About John Walton
Aviation journalist and travel columnist John took his first long-haul flight when he was eight weeks old and hasn't looked back since. Well, except when facing rearwards in business class.