I love your country. Been there in '96 for six weeks. I bought an old Ford Cortina wagon 4 liter and drove from Sydney through Alice Srings up to Cairns. Only one waterhose went in the middle of nowhere, but I was prepared...
Yep, a sparkplug failure can make your car run rough on idle. Even cause to fail as the management system does not understand the lack of flexibility and large emission. So it adjusts it's settings incorrectly.
You could take them out one by one and check the spark while a friend starts the car and you hold the spark plug (wire connected) to the engine block with isolated plyers. Or have the car running at idle and remove and reinstall the spark plug wires one by one: on the cilinder where the car does not start running even worse, is the location of your problem..
Change all six of them once a year: will save a bit of fuel, thus making up for the cost. It's an easy DIY job. Just one at the time and do not use force installing the new ones. Have a bit of copper grease handy for the tread.
Buy a Haynes manual on your car: it has a section on spark plug reading and other maintenance things like changing oil and filters.replacing belts, adjustment of valve clearance (has yours hydraulic lifters?) etc. etc. Do not forget that a well maintained car is reliable and wil run forever: cars mostly break down because of lack of maintenance.
Did you ever change the automatic fluid, or do you wait until the trans makes funny noises and the trans repair guy wants 1000$...
P.S. If you have one of those nice infra red temperature measuring guns and your exhaust ports are uncovered, you could check whether the temperature is the same at each cilinder, if not you found the location of the problem.