There are going to be as many answers as maltsters, as per Troopers comment I asked on a tour of Coopers a couple of years ago about Schooner and was told that they at that time no longer used exclusively Schooner. They used to own Adelaide Malt and make their own malt but that was sold to Jo White years ago. Jo white still run (or did last time I looked) a maltings in Adelaide primarily for Coopers and do make malt to Coopers specifications - that is what will be used mainly in their extract.
Black Rock says that their malt extract was made from 100% NZ Lager malt.
Muntons and the other reputable extract makers will be using what is local to their works. After that it can get pretty hazy, some of the low priced extract carefully avoids using the word Malt as it is manufactured enzymeically from what ever is cheapest. There was a report that one reasonably well known kit maker in the US was found to have made kits without any malt extract in them at all, sugar caramel and some "syrups" but no Malt extract.
So I guess you pay your money and take your chances.
That said I have found the malt available in Australia from good suppliers to be excellent. I did get sent a sample of "Malt Extract" (DME) from China that on evaluation I wouldn't put in beer, I just hope no one decided to import that crap.
Mark
Mark