OK
http://www.actionaluminium.com.au/index.html They have interstate branches! QLD has the bigest stock/range.
Give them a call and see what they can do for you. In Melb they are quite friendly.

Here is the trick.. If you are thinking of making production run parts think about making jigs and the attachment of those jigs.
If on the other hand you intend to have a big plate with dowels and threads then it is a lot of money and rarely used to its full capacity.
I did a similar design that had a series of pedestals that supported a number of plates.
At the end of the day we ran a dial mic over the jig and adjusted the jig so as to have finished parts within tolerance.
Moral of the story .. figure out what you want to do and design to meet those needs. bolt it on so as to have flexibility if the goals move or change. example " oh dear a plasma cutter would be nice"
Now we need a fume extraction or water bath as a bed.
I don't mean to confuse the issue, a good plan and a set of goals is more that half the work.