+1 for vroomvroomvroom. To avoid the airport premium do a search using the city location as a pick-up to compare to the airport pick-up. I have found on a number of occassions a much cheaper alternative is get myself into town via cab or bus, hire car and then have the drop off set for back at the airport. As your friends are flying long haul into OZ I would also recommend not picking up the car straight away.
Sometimes downtown has a similar fee, ie this is an old hertz listing:
Location Fee
Adelaide Airport 14.00 %
Alice Springs Airport 17.00 %
Armidale Airport 6.00%
Avalon Airport 10.00 %
Ayers Rock Airport 19.00 %
Ayers Rock Downtown 19.00%
Ballina Airport 5.00 %
Brisbane Airport 14.00 %
Broome Airport 10.00 %
Cairns Airport 14.00 %
Coffs Harbour Airport 4.00 %
Canberra Airport 22.00 %
Coolangatta Airport 14.00 %
Darwin Airport 14.00 %
Emerald Downtown 15.00%
Emerald Airport 15.00%
Geraldton Airport 5.25 %
Grafton Airport 6.00%
Gunnedah Airport 6.00%
Hervey Bay Airport 10.00 %
Hobart Airport 12.00%
Kalgoorlie Airport 10.00 %
Karratha Airport 9.22 %
Launceston Airport 7.50 %
Lismore Airport 2.00 %
Mackay Airport 11.00 %
Maroochydore Airport 14.00 %
Merimbula Airport 5.00 %
Melbourne Airport 17.00 %
Mildura Airport 5.00 %
Moree Airport 6.00 %
Moruya Airport 5.00 %
Mt.Isa Airport 11.6%
Narrabri Airport 6.00 %
Newcastle Airport 16.00 %
Perth Airport 15.00 %
Port Hedland Airport 9.22 %
Port Macquarie Airport 3.30 %
Proserpine Airport 5.00%
Rockhampton Airport 13.50 %
Rockhampton Trucks 13.50%
Sydney Airport 22.00 %
Sydney City 5.00%
Tamworth Airport 6.00%
Taree Airport 6.00%
Townsville Airport 16.00 %