Travelling from London Via Guangzhou to NZ - 72hr Visa Free Entry

I hold a british passport and am travelling from London to NZ via GZ, I have a direct flight to NZ with a stop over in Baiyun Airport of 7hrs, can we leave the airport to look around the city for a few hours, do I need to apply for this Visa Free Entry before I leave London, or can it be done when we arrive at the airport?? Many thanks