A continent is made up of a large, thick slab of continental crust. Whether it's above water or not is immaterial.
The main author of the article, New Zealand geologist Nick Mortimer, said scientists have been researching data to make the case for Zealandia for more than two decades.
"The scientific value of classifying Zealandia as a continent is much more than just an extra name on a list," the researchers explained.
"That a continent can be so submerged yet unfragmented" makes it useful for "exploring the cohesion and breakup of continental crust".