Hi Winston, Manapouri was built by Bechtel concurrently with Tiwai Point, the government at the time never intended the power to go elsewhere. If Tiwai were to close suddenly, up to 400MW of Manapouri output would be stranded without the transmission being upgraded (existing corridors) through to the HVDC terminal at Benmore.
http://www.3news.co.nz/Grid-can-cope...3/Default.aspx
As I understand there is a four year notification period, so the risk is that Transpower may not be able to get the transmission upgrade built in that timeframe, politically and/or physically. Sounds like a long time, but NIMBY's don't like transmission lines.
The 630MW potentially available (inc the stranded 400MW) from Manapouri would go into the market, likely displacing thermal output at Huntly, so there are risks to both Meridian and Genesis.