What will happen when we run out of number plate letters?

What's going to happen when we run out of letters in the alphabet for car number plates?

That's what a Freedom of Information request is asking, saying 'TMN' is now being used as the starting letters in registrations. 

In response, the Department of Infrastructure says it's not thought about the issue yet. 

The DOI adds the current series of registrations will take around five years to work through -  but admits it still needs to decide about future plates.