Whyalla is under siege by up to 800 well-equipped aggressors seeking to destabilise the regional South Australian town.

At least that is the brief for some 9,000 Australian, New Zealand, United Kingdom and United States soldiers working to evacuate Australian expats and foreign nationals before they re-establish security in the city.

It is a fictional scenario taking place in a fictional country called Cultania, but heavily armed forces, responding via invitation by the "Government of East Cultania", are taking it very seriously.

I was embedded with the Fifth Battalion Royal Australian Regiment on Tuesday as they participated in the largest military exercise South Australia has ever seen, Exercise Hamel.

A regional city under threat

1400: I arrive at Battle Ready Headquarters at Whyalla airport where I am given a briefing by Lieutenant Colonel Paul Shields. An active terror base organisation called Islamic State of the Archipelago of Cultania (ISPAC) is seeking to establish control of Whyalla and impose Sharia law. ISPAC is backed by a state-based, high-end conventional military force amassing on the border just 70 kilometres away. Phase one of the operation is to evacuate expats. Phase two is to regroup with combined international forces and re-establish security around Whyalla and clear it of aggressors.

1445: New Zealand soldiers arrive by Chinook helicopter.

Evacuation of expats underway in city centre

1515: I am transported to the Evacuation Handling Centre operated by Combat Ten Bravo in the city's centre. Security checks are thorough and repetitive. My bags are searched twice and my knife temporarily confiscated as my status as an ABC embed is confirmed.

Officer Commanding Major Maj Trembath says the evacuation centre has received a steady influx of people wanting to leave due to the high-security threat posed by ISPAC.

"They may want to stay if they've got a business or whatever," he said.

"At the end of the day ... most people would come here if there's an invitation."

1540: I am processed and sent into a holding area with other evacuees. Many are frustrated after waiting hours for transport to the airport. Waiting evacuees were earlier confronted by about 15 angry locals outside the fence shouting abuse and pushing on the barbed wire. They were angry with the expats for "taking their jobs". Australian soldiers segregated them from the expats and calmed the situation.

Armoured transport to airport

1600: A fleet of Protected Mobility Vehicles — Bushmasters — arrive to transport about 36 waiting evacuees to the airport. Heavily armed soldiers man the turrets.

1620: I arrive at the airport where evacuees are searched again and processed. Department of Foreign Affairs and Trade officers undertake thorough checks to ensure they are Australians and approved nationals.

1800: I am transported back to the evacuation centre and attached to one of several satellite patrols that will maintain security overnight. This will include questioning people walking the streets at night. We march through Whyalla towards a secure area inside an apartment block serving as a central location for the patrol. A patrol is already out and is due to arrive back soon. My patrol will push out at 2200 and continue until 0200.

Night patrol through Whyalla streets

2000: Shots fired. Radio communications report multiple rounds fired back at the evacuation centre. Patrol members prepare to respond on orders. Investigations reveal the burst of automatic fire appeared to be a drive-by shooting.

2145: I return to the evacuation centre. I am informed on arrival a small riot had taken place and soldiers in the centre were threatened and heckled aggressively by locals. Soldiers controlled the situation with the assistance of military dogs and the aggressors left, warning they would be back during the night.

2230: I bunker down on a stretcher as patrols come and go throughout the night. No activity is reported.

0700: Scores of more people arrive for evacuation. I depart for Adelaide.

Certification for Battle Ready status

Exercise Hamel is a training operation for the Fifth Battalion, based at Robertson Barracks, Darwin. After Exercise Hamel, which includes humanitarian assistance missions, disaster relief operations and high-level war fighting scenarios, these soldiers will be considered 'Battle Ready' and can be deployed overseas.