Australian Federal Police Deputy Commissioner Michael Phelan said on Friday that an overseas commander within the so-called "Islamic State" terror group had "inspired and directed" two men in their aborted plot to plant a bomb on an Etihad Airways flight out of Sydney on July 15.

"This advice was coming from a senior member of the Islamic State," Phelan told a media gathering. Police claimed the IS leader had sent components to be used in the improvised explosive device (IED) through the international mail from Turkey. The terror leader also instructed the men on how to build the "military-grade explosive."

Aborted at the check-in counter

According to police, a 49-year-old man brought the bomb, disguised as a commercial meat mincer, in a bag to the airport, where he passed it on to his brother without telling him about its explosive contents, Phaelan said. However, the bag was never checked in; the 49-year-old who delivered the bomb left the airport with it once again in his possession while his brother went on to board the flight.

It remains unclear why the bag never made it on to the flight as intended.

Australian police arrested four men on July 29 in relation to the Etihad Airways terror plot

Police have leveled terror-related charges at two men involved in the plot, one of whom is the 49-year-old. His brother has not been charged. The other is a 32-year-old man. No names have been released.

The two suspects are due to appear in court on Friday. They were among a group of four men arrested during raids in Sydney on Saturday. One man remains detained for questioning, and the other one was released without charge.

Phaelen called the bombing plot "one of the most sophisticated plots that has ever been attempted on Australian soil".

Plans for poison gas

The police official also alleged that the two men developed a plot involving a "chemical gas dispersion device" that would release the poisonous gas hydrogen sulphide.

Pheland said the two charged men turned their attention to a chemical gas attack after the aborted plane plan

"Not only have we stopped the IED that was believed to go on the plane, but we have also completely disrupted the intended chemical dispersion device," Phelan said. "Hydrogen sulfide is very difficult to make, so I want to make it quite clear that while it may be a hypothetical plot, we were a long way from having a functional device," he added.

A location for the chemical attack had not yet been chosen, although the IS leader had provided suggestions as to where a dispersion device could be placed.

Australian police received a tip-off on July 26 alerting them to the ongoing terror plans, Phaelen said. They subsequently arrested the four men on July 29.

Etihad is assisting the police with the ongoing investigation.

Watch video 00:32 Australian police disrupt plot to down plane

cmb/gsw (Reuters, AFP, AP)