Online, theories abound about the cause of contrails, most involving governments covertly spraying chemicals onto an unsuspecting populace.

But Rebecca Kamitakahara, forecaster with the Bureau of Meteorology in Canberra, said the phenomenon, also known at jet trails, could be explained simply.

Ms Kamitakahara said contrail was an abbreviated term for condensation trail. The long, straight lines in the sky are triggered by the condensation and freezing of water vapour from the exhaust of aeroplanes.

The upper atmosphere above the capital has been particularly wet over the past couple of days, which has meant the tiny ice particles hung around in the sky rather than evaporating.

Canberra lies on the Melbourne-Sydney flight path, the busiest in Australia and one of the busiest in the world and according to Bureau of Infrastructure, Transport and Regional Economics, more than 704 000 passengers flew between the two cities in August, an increase of 9.5 per cent on the same period last year.

More than 280,000 passengers flew in and out of Canberra in August, making it the 8th most utilised airport in Australia.

Little wonder the sky has been criss-crossed with what looks like long white clouds.