Passengers on a Qantas flight that landed in Sydney on Friday morning have been warned that a fellow passenger was infectious with measles.

The man in his 20s travelled on Flight QF20 from Manila, which landed at Sydney Airport at 6.30am on January 11, NSW Health said.

Measles is one of the most infectious of all communicable diseases and is spread by tiny droplets generated through coughing and sneezing.

He developed measles while he was in the Philippines and was diagnosed with the highly infectious disease after returning home to Sydney.

It is the sixth case of measles reported in Sydney in the past three weeks - all due to people returning, or visiting, from countries where measles is prevalent.