When can you find hidden DexNav Pokemon in ORAS?

Question

Is it possible to find the hidden DexNav Pokemon (pokemon that don't usually appear on the DexNav, for example, Zorua on route 101) before defeating the elite four? Can you catch them as soon as you receive the DexNav?

Answer 1

You can't get them as soon as you receive the DexNav, but you can before the Elite Four. It's possible to find the hidden DexNav Pokémon when you finish the story with Primal Groudon/Kyogre. You just need to finish that bit of the story, and then go back to Professor Birch. He will then give you the National PokéDex.
̶e̶d̶i̶t̶e̶d̶ ̶b̶e̶c̶a̶u̶s̶e̶ ̶I̶ ̶r̶e̶a̶d̶ ̶t̶h̶e̶ ̶q̶u̶e̶s̶t̶i̶o̶n̶ ̶w̶r̶o̶n̶g̶

Source: Serebii

Comments

"before defeating the elite four? Can you catch them as soon as you receive the DexNav?"

This is factually wrong.

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You also don't actually need to upgrade the Pokedex, either.  Though I'm not sure why you would want to NOT upgrade it...