In order to reassure Duke Sergei and also to encourage the Tsardom of Russia behind him to be generous, Arthur took Duke Sergei to visit the largest shipyard in Australasia, the Sydney Royal Shipyard owned by Arthur himself.
The construction plan for Australasia's navy moved quickly, and they had already begun building small and medium-sized warships.
Indeed, Australasia chose to build small and medium-sized warships first, leaving the most expensive dreadnoughts for later.
In fact, it wasn't really leaving them for later; the main issue is that Australasia's dreadnought designs were still only theoretical at this point. Building a real dreadnought would still bring up many issues.
Thus, it requires a test unit to help the shipyard's experts and employees become more familiar with the construction of dreadnoughts, and Arthur would naturally not build an extra test unit.