Traveling from USA to Australia, for my phone do I need a power converter, or only an adapter plug?

The converter is built into the adapter.

Theoretically you should be able to buy a proper Samsung wall plug in Australia which will mean no need for the adapter.  

Unless your hotels has USB sockets you'll need to adapter for sure.

Look at the power plug for your Phone and read the tiny print. INPUT VOLTAGE. Most of them say 100-240VAC.

If it says that your ok.

You'll only need a plug adapter for electronic devices like a phone, camera or laptop. If you want to be absolutely positive, just check the devices/chargers and on there it will give you a range of 110V and 250V (or similar).

You'll only need a power adapter for high powered devices like hair dryers or straighteners and even then, it's worth checking as a few a dual voltage. If you'll be staying in hotels or decent motels, most have a 110V outlet so you probably won't need a power adapter at all.

You will only need a plug adapter to fit our power points, the phone charges adapt to the voltage.

It depends.  You only need an adapter if you're only taking things that charge via USB or have built-in adapters.  The USB charger that comes with most phones and electronics is designed to work with both voltages and will convert either of them.  The same is true of most camera battery chargers.  I spent two weeks in Australia and only used an adapter.  I had an iPhone, a Kindle Fire, a Kindle Paperwhite and a Nikon camera battery charger.  My husband's Samsung charger worked fine as well.

You only need a converter if you have something that is designed to only work with the US 110 current.  For example, you'll need a converter if you plan to use a hair dryer or curling iron.  

why don't u just buy one from a gas station when u get there