A “mega blaze” raging across a 60km (37 mile) front north of the Australian city of Sydney cannot currently be put out, fire officials have warned.

The blaze across 300,000 hectares (1,150 sq m) is just an hour’s drive from the nation’s most populous city.

People who cannot defend their property from approaching fires have been told they should leave immediately.

Since October, bushfires have killed six people and destroyed more than 700 homes across Australia.

The severity of the blazes so early in the fire season has caused alarm and prompted calls for greater action to tackle climate change.