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As stated in Timatic, the database used by airlines:
Nationals of any country can obtain a visa on arrival at Nouakchott (NKC).
In addition, various forums, such as this, have anecdotal reports of it being possible to obtain a visa on arrival at land crossings too.
There is no Mauritanian embassy in Australia; as such, unless Tokyo accepts applications by regular mail, there is no option for getting it in advance while in Australia.