New Zealand Rugby chief executive Steve Tew says it's too soon to talk about Japan joining the Rugby Championship.

Tew said Sanzaar's current focus is on improving the competitiveness of the Sunwolves in Super Rugby.

'At some stage Japan being considered for the Rugby Championship would be on the table but, clearly, that has to come at a time when they were able to compete,' he told the New Zealand Herald on Thursday.

'Right now Sanzaar is focused on ensuring the Sunwolves can compete in Super Rugby. That's the first step, and they've clearly got a way to go.'

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