SYDNEY: Australian police said two men have been charged over terror-related offences involving plans to place an improvised explosive device on a passenger jet flight leaving Sydney, and, separately, to build a device to release poisonous gas.

Australian Federal Police Deputy Commissioner National Security Michael Phelan told a media conference on Friday that the men had assistance from Islamic State in Syria over the plot targeting an Etihad Airways flight.

The explosive device was taken to Sydney's airport but the plan was aborted and the bomb did not breach airport security, he said.

"This is one of the most sophisticated plots that has ever been attempted on Australian soil," Phelan said.

"The explosive was a high-end explosive...I don't want to be specific because it's still under examination for the exactness of it, but high military grade explosive."

Phelan said in a separate event, the same men attempted to create an improvised chemical device, although he said there was no evidence to suggest that would be used in a plane attack.

Police arrested four men last weekend in raids across Australia's biggest city of Sydney. One man has been released while another is being held without charge under special counter-terror laws.

Etihad said this week it was assisting police with its investigation.