If you're not sure what these Australium weapons are, look at todays' TF2 blog posts:
Day 1 and
Day 2.
Basically, these weapons give cool effects on the wearer. Here's how you get them:
The first of our Two Cities, Mannhattan, takes our mercs all the way from the sun-blasted gravel pits of the Badlands to the urban sprawl of the East Coast, where they'll have to defend the manufacturing arm of Saxton Hale's boutique Mhankö line. Mannhattan's robot hordes can advance their own spawn points and introduce new bombs into the map for the first time, making for a unique new MvM experience. The loot you'll get from completing a Tour of Duty will work a bit differently for the Two Cities Tour. Completing a Two Cities Tour automatically grants you a Killstreak Kit, which you can apply to any weapon to transform it into a Killstreak weapon. Completing the Tour also grants you a fabricator and spare parts that you can use to craft progressively rarer Killstreak Kits, which will add cool visual effects to your weapon and eventually even your character.
Finishing a Two Cities Tour might even get you one of the most exceedingly, some might say preposterously rare items in the game: the Australium weapon. (Hushed silence. Then, from the back, a lone slow clap. Then: thunderous applause.)
Yes, that means you need to
farm complete MvM Two Cities tours. Tours if you remember from the first few MvM patches are collections of MvM missions on multiple maps. You need to buy a Tour of Duty Ticket from the store (or trade) and can only be played on Valve's MvM servers, so called Mann Up mode. Previous Tours granted Strange Botkiller weapons.
Completing this new Two Cities Tour will grant you a Killstreak Kit. That Kit can be aplpied to any weapon to transform it into a Killstreak weapon. You also get a fabricator and spare parts, and you should be able to craft rare Killstreak kits (hence the farming). At some point (and if you're lucky), you can find very rare items: these Australium weapons from Mango's post. It's going to be interesting to see if they can be sold on the Steam market or even traded...
Now, Mango asked a question earlier:
"ApatheticMango: New MvM maps. New upgrades. Australium Weapons. Chance for an MvM server for a short time?" We used to have 2 MvM servers back when the mode was first released. It was fun, for a while. And then Valve decided to add special loot (the botkillers) with their Tours of Duty and Mann Up mode. Problem: Mann Up mode is restricted to Valve servers. We can't run our Mann Up servers. Our MvM servers don't give special loot.
Also, an MvM server requires as much resources (CPU/RAM) as a full 32 slot server. It obviously only hosts 4 actual players. There just wasn't any good reasons to keep these MvM servers: too expensive, not enough players, and not even Mann Up mode.
I don't think anything changed in today's update. It seems if we want to get the new weapons/kits, we have to play on Valve's MvM servers. I don't honestly see a reason to host our own MvM servers right now.
I might be wrong though, I haven't played MvM in months and I never bothered with Tours of Duty. Do you guys know more about this whole thing?