Going to the city, you can take the airport shuttle, it costs AUD15 per single trip. When you leave the airport, then go to left hand side, you can see the shuttle bus waiting area.
For return trip, it has discount, total AUD28. You should be advised that there is no fix time table posted on it, so you have to use the phone at the waiting area for calling the shuttle.
I and my friend, together with a couple (total 4 persons) go the city by taxi , the price is same (i.e. each has to pay AUD15). Please be reminded that you should ask the price to the taxi driver before get on. So if you have 5 persons, you are better take a taxi.
If no traffic congestion, it normally takes 30-40 minutes to the city.