(snip)

They are called number plates - the term "licence plates"  is an unwanted
Americanism that has only appeared in the last twenty years. The plates are
issued by the relevant State authority (except WA used to be the local
government, may still be) when the vehicle is registered for use on the
road, and must be returned to them when the registration is allowed to
lapse, which may be when it is scrapped, moved interstate, or taken off the
road for any reason. Numbers may be reissued to another vehicle at the
authority's discretion, or the registration may be transferred to your new
car.
About fifty years ago the states came to an agreement where the then
existing systems of numbers (which varied, but typically were either a
simple number, up to six digits, or two letters followed by three numbers)
was replaced by three letters followed by three numbers, with the alphabet
of the first letter allocated to the states in proportion to the state
population, and additionally plates from adjoining  states different
colours. For example, NSW had A-F, black on yellow, Victoria G-M, white on
black, Qld N-Q, white on black.  Z followed by a four digit number was
allocated to Federal government cars. (Most states have a separate system
for trailers and motorbikes)
This system ran out of numbers, even reusing cancelled numbers, quite a few
years ago, and most states simply continued with the same system, but using
letters which were originally allocated to other states (The number plates
have always been marked with the state name or abbreviation). Victoria
changed the colour to green on white and restarted from A, Qld changed to
putting the numbers first and started from A.
Just last month, NSW, where I live, ran out of numbers at ZZZ-999 and
restarted a new system with two letters, two numbers, two letters.
In addition, over the last twenty years all states have started selling
premium plates, where for an additional fee you can have a number plate not
in the regular sequence - and the variety of these seems to be limited only
by your imagination! (plus what others already have)

The number plates are a standard size (usually motorbike and sometimes
trailer plates are smaller), and this standard causes problems with many
imported cars. For example, they are about half an inch too wide to fit
next to the back door or tailgate  on a S2/3/90/110 and too high to fit in
spaces designed for the English height numbers. (I think the digits are the
same size, but there is an allowance for the state name at the top plus a
border.)

See my earlier post about trailers. As you can see from the above, the
registration number may or may not convey anything about its age, and may
change during its life. For example, my 110 has had three numbers in its
life. It was first registered in the Northern Territory, then sold to
Victoria, where it acquired a new number, which it retained when I bought
it, but then changed again when I moved to NSW.
Hope this clarifies matters.
JD

I'm not sure how it works in the USA, so I can't really compare.  We
call them either vehicle registration plates or number plates and
they're attached to the vehicle when it's registered for use on public
roads.

Here, a licence is a permit, issued for up to 5 years, which authorises
  a person to drive a registered vehicle on the road.  Our license has
no relationship at all to the number plate.