In the GSA Today article that Michael linked to in the comments, it says:
The Glossary of Geology defines a continent as “one of the Earth’s major land masses, including both dry land and continental shelves” (Neuendorf et al., 2005). It is generally agreed that continents have all the following attributes:
- high elevation relative to regions floored by oceanic crust;
- a broad range of siliceous igneous, metamorphic, and sedimentary rocks;
- thicker crust and lower seismic velocity structure than oceanic crustal regions; and
- well-defined limits around a large enough area to be considered a continent rather than a microcontinent or continental fragment.
The first three points are defining elements of continental crust and are explained in many geoscience textbooks and reviews (e.g., Holmes, 1965; Christensen and Mooney, 1995; Levander et al., 2005; Kearey et al., 2009; Condie, 2015). To our knowledge, the last point—how “major” a piece of continental crust has to be to be called a continent—is almost never discussed, Cogley (1984) being an exception.
and goes on to mention:
Furthermore, the 4.9 Mkm^2 area of continental crust is large and separate enough to be considered not just as a continental fragment or a microcontinent, but as an actual continent—Zealandia. This is not a sudden discovery but a gradual realization; as recently as 10 years ago we would not have had the accumulated data or confidence in interpretation to write this paper.
This part is important too:
spatial and tectonic separation, along with intervening oceanic crust, means that the Zealandia continental crust is physically separate from that of Australia. If the Cato Trough did not exist, then the content of this paper would be describing the scientific advance that the Australian continent was 4.9 Mkm2 larger than previously thought.
So to wrap it up, they say:
Being >1 Mkm^2 in area, and bounded by well-defined geologic and geographic limits, Zealandia is, by our definition, large enough to be termed a continent. At 4.9 Mkm^2, Zealandia is substantially bigger than any features termed microcontinents and continental fragments, ~12× the area of Mauritia and ~6× the area of Madagascar. It is also substantially larger than the area of the largest intraoceanic large igneous province, the Ontong Java Plateau (1.9 Mkm^2).