http://lmgtfy.com/?q=how+do+nz+tv+ratings+work

Sky might be able to if you connect your decoder to the internet, as they aren't exactly forthcoming about what they do and don't track and datamine.

For the rest it's survey-based. Over-the-air TV broadcast is one-way, and without a return data path they have no way of tracking you.

In America they used to drive around in detector vans listening for signals from TVs in the houses they passed. Receivers also transmit weak heterodyne signals that can be used to determine the channel they are tuned to. 

No, I mean the USA. There was no license fee there because of the advertising but they used detector vans for survey purposes.

The UK claimed to do that, but most people thought the vans were a ruse. Anyway these days there would be far too much RF around, and the IF so small because it never leaves the single chip reciever that I doubt it would work at all.

They give a select few people a box that they have to input who is in the room, and what is watching. Last person I know that had one it was back in the CRT Tv days and it had something to tell what channel the TV was on, I know that the later ones would plug into a sky box somehow, but no idea how they worked. People have to keep logging in when they are in the room watching to the box. May have changed now tho. No idea how it would cope with timeshifting or on demand etc viewing. The on demand would be accounted for by the streaming servers anyway.

richms:

The UK claimed to do that, but most people thought the vans were a ruse. Anyway these days there would be far too much RF around, and the IF so small because it never leaves the single chip reciever that I doubt it would work at all.

They give a select few people a box that they have to input who is in the room, and what is watching. Last person I know that had one it was back in the CRT Tv days and it had something to tell what channel the TV was on,