I think the reason Zealandia is called a “submerged continent” is because it is made of continental crust rather than oceanic crust.
But IMO the best geologic definition of continents is by tectonic plates, which mostly matches up with the cultural definitions of the continents.
For the major continents, we have these plates:
North American
South American
Eurasian
African
Australian
Antarctic
There are several smaller plates too, like the Caribbean, Indian, and Arabian plates. IMO, we should consider these independent continents.
There is also a dedicated Pacific plate. The ring of fire is the border of this plate.
New Zealand / Zealandia is on the ring of fire. Half on the Australian plate, half on the Pacific plate. You can actually see the border of the two plates when you look at the topographical map of Zealandia.