It's unlikely returning New Zealanders are to blame for rising median house prices.

Real Estate Institute figures show median house prices across the country rose by nearly 20 per cent.

In Auckland it rose by 16.3 per cent to hit a $1 million.

New Zealand Initiative senior fellow, David Law, told Andrew Dickens new migration data shows net migration plummeted to 2,500 people in the six months to September.

"That's about 12 times less than for the same period last  year - so it seems unlikely net migration is having an effect on house prices."

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