"We will provide a number of military platforms, up to eight Super Hornet aircraft to participate in a US-led coalition in delivering air strikes," he told reporters in Baghdad after meeting Iraqi Prime Minister Haider al-Abadi.
Prime Minister Tony Abbott announced on September 14 that Australia would send fighter jets and forces to the United Arab Emirates as its contribution to the US-led military effort against the Islamic State (IS) jihadist group.
"We are some distance from the start of that now, we need to deal with a few technical matters with respect to operations," said Johnston.
The deployment also includes an E-7 Wedgetail early warning and control aircraft and an aerial refuelling tanker aircraft, as well as special forces who will act as military advisers.
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The announcement comes after Australian police last week foiled an alleged plot by IS jihadists to conduct "demonstration killings" in the country, including randomly beheading members of the public.
According to the Pentagon, US aircraft have carried out 178 air strikes against IS targets in Iraq since August 8. France has since joined the campaign and launched its own strikes.
During his meeting with Johnston, Abadi reaffirmed his "rejection of any ground intervention in Iraq," his office said.
Washington has repeatedly asserted that it would not deploy ground troops to the country in which its forces fought a bloody and costly war before withdrawing at the end of 2011.
Footage of the beheading of foreign hostages, mass executions of Syrians and Iraqis and the scale of the humanitarian disaster caused by IS has shocked public opinion.
There was particular revulsion in Australia last month over a picture of what is believed to be the young son of an Australian man holding the severed head of a Syrian soldier by the hair.