I just tracked two of the kits down online via the Billing website, www.billingboats.com. It contained a link labeled "order online" (or something similar). When I clicked on that link I got routed to the site of an American online hobby shop called Tower Hobbies (www.towerhobbies.com), which had the Oseberg kit for $69.95 (U.S.). If you tried the same trick in Australia, maybe you'd get routed to an Australian dealer.
The Billing site does list an Australian distributor, but he only operates as a wholesaler. A local hobby shop probably could order from him.
The company actually offers two kits labeled "Oseberg," both supposedly on 1/25 scale. (I think that may be a typo, though; it looks like one is considerably bigger than the other.) The second one, called "Mini-Oseberg," apparently is intended for beginning modelers. That might be a reasonable way to get one's feet wet in the hobby. Both are based on the Oseberg Ship, one of the two surviving actual Viking vessels. (The name would have meant nothing to the original owners. It's the name of the place in Norway where the ship was found. I know of no evidence that the Vikings - or anybody else in medieaval Europe - assigned individual names to ships.)
I haven't built or even seen either kit. On the basis of the pictures on the website, though, the non-mini one does look pretty much like the Oseberg Ship. The pictures are too small for me to see the bow and stern ornamentation clearly, but they seem to be about the right shapes. Be advised, though: the real Oseberg Ship doesn't meet the definition of either "longship" or "trader." It's far too small for either purpose, and it has so little freeboard that the experts think it probably wasn't a seagoing vessel. The best guess is that it served as some sort of ceremonial yacht, probably for the aristocratic woman who was buried in it.
I'm a little uncomfortable about Billing's description of the vessel's history, which refers to the depth of water in which the ship was found. Unless I'm badly mistaken the Oseberg Ship was found in a burial mound on dry land. And I have doubts about the shields that Billing has arranged along the gunwales. This wasn't a fighting ship - though I suppose its occupants might have brandished shields for ceremonial purposes.
The Ege seems to be a replica of one of the ships found at Roskilde. I don't know much about that find, but based on the photo the model certainly looks believable. The Roskilde vessels, if I remember correctly, are quite small - not longships by any stretch of the imagination, but the model certainly does have that beautiful Scandinavian look about it. It's a little cheaper and, according to Billing's star system, considerably easier to build than the non-mini Oseberg Ship.
I couldn't find the third kit you mentioned. Maybe it's been discontinued; the name doesn't ring a bell.
Hope this helps a little. Good luck.