Plan to Australia in March,可否提供行程

There is no best itineary. My best arrangement doesn't equal to yours. So, I can only give some some suggestion.
I don't know if you have bought the air ticket or not. If no, I suggest you flying to Sydney first, then Gold Coast, and finally Cairns.
You may spend around 3 days in Sydney. 1.5 day in the city centre and another 1.5 day in it's surroundings (e.g. Taronga zoo and Blue mountain etc)
Then you can fly from Sydney to Gold Coast directly and spend 3 - 4 days there. You may enjoy the surfing, hot air balloon, or even shopping in the factory outlet.
Another 3 - 4 days will be in Cairns. 1 day going out the the Great Barrier Reef, 1 day going to the Kuranda village (Rainforest and cable car), 1 day for another activities (like fishing etc.)
Finally you can fly back from Cairns to HK directly if you are flying with Cathay Pacific, otherwise you may need to return to Brisbane for fly back

2008-02-22 12:35:27 補充：
From Sydney airport, you can simply take the train to Sydney Central Station. It's the heart of the city, then Darling Harbour is only around 10 mins walk from there.

2008-02-22 12:35:44 補充：
If the cost of ticket from Cairns to GC is too high, then you may consider flying to Brisbane, where is only 1.5 hours away from GC. The is coach running bewteen Brisbane airport and GC.