As Waylander says, Japan has no problem with buying European. E.g. AW101 helicopters, Olympus & Spey marine gas turbines, radar technology from Thales, Adour jet engines, FH70 155mm howitzers,various European small arms, Carl Gustaf, Panzerfaust 3, French & British mortars, Otobreda naval guns, British 105mm tank guns . . .
Sure, its not impossible.Japan could deal with the Norwegians directly as they have done with various European equipment and designs mentioned.
But stealthy cruise/maritime strike missiles are in a different category, and have a wider strategic implication. If Japan is serious about an alliance then this might be the type of project it makes sense to partner with Australia. I don't believe Japan currently manufactures Harpoon, and buys from the US. I'm not aware of any missile Japan currently fields that is European produced or sourced they seem to be indigenous (usually off US designs or concepts) or US based.
So going against Harpooner or JSM, as a F-35 operator in a high threat enviroment, against a capable foe, JSM would seem to be pretty attractive.
Specifically because Australia has developed parts of the JSM/NSM missile like the radio seeker and funded F-35 development, Australia isn't a passive purchaser in this missile. Australia is likely to continue funding and developing improvements for this particular missile and updating/mapping relevant possible threats and making various improvements. I am not aware of the details, but I would assume Australia has some IP in the JSM version.
While it would be great to have a clean sheet indigenous missile tech pouring out of Australian factories from Australian companies, JSM offers a very real, slightly shared development. The JSM finds itself in a niche, due to Australian funded F-35 development which makes it very attractive to other nations like South Korea and Japan. Where JSOW/Harpoon doesn't have compelling advantages.
I see Choules has left Sydney and is able to be there for support up north and the others will be available in an estimated 10 days. It seems to be preventative on Adelaide. I am sure Choules will be able to provide what the ADF needs in Queensland.
Again though, lack of amphibious (in this case HDAR) hulls is again an issue. Good thing it was spotted before Talisman.