Former Prime Minister John Key has reportedly sold most of his sprawling Parnell property to a buyer in China.

NZME reports Mr Key and wife Bronagh pocketed around $20 million, keeping only around 650 square metres of the original 2340 square-metre site.

It's believed the couple are planning to build a smaller house where the tennis court currently sits.

The Keys didn't respond to a request for comment.

Newshub.