Due to the established diplomatic relations between Australasia and the major European powers, it was sufficient to notify them by summoning their foreign ministers.
Among these European powers, Australasia specifically invited the influential minisiters of England, Germany, as well as Russia, which had a very good relationship with Australasia in the international community.
The response from the powers came quickly. On September 30th, Russia responded, expressing its willingness to participate in the mediation as a neutral country and strive for peace between Australasia and Portugal.
Germany and England followed suit, agreeing to participate in the mediation. Germany had no involvement in the war, and the conflict was far remote from German territory.
Germany's willingness to participate in the mediation was actually due to Wilhelm II granting Arthur a favor.