This page (down the page a bit) shows you're right. But the second highest is the morning peak, and that graph is averages. In this case it's an unseasonably cold morning across NZ so for this day in particular they are worried about everyone using their heating at the same time, their forecasts are probably for the morning to have a higher peak on this day in particular.
The article got updated to say that capacity is down at the moment because of pre-winter maintenance, and the unseasonably cold weather means they weren't prepared for this.