In the very long term you may be right. New Zealand's demand is growing at around 2% per annum. That equates to an increase in peak demand of around 130 MW per year. So in a less than 5 years the national peak demand will have grown by around the amount consumed by Tiwai, and in around 7 years the average demand will be there as well.
However looking at averages just hides the situation. We can still not get that extra energy north of Roxburgh due the power lines just not being big enough. Transpower have announced that they are fast tracking their CUWLP (Clutha Upper Waitaki Lines Project) and aiming for a completion date of May 2022. That will still not allow for all of that energy to be transferred north.
"
CUWLP will increase capacity on transmission lines between Clutha and the Upper Waitaki Valley so additional generation can be exported from Southland. Its completion would enable about half of the electricity now used by Tiwai to be transmitted to Cook Strait."
From Energy News
https://www.energynews.co.nz/news-st...e-now-may-2022
I can see that there will be some strange pricing signals over the next 5 to 7 years. The stranded energy in the lower South Island will cause wholesale prices there to fall dramatically. The extra transfer over the HVDC will see extra reserve requirements in the North Island and that will push up North Island whole sale prices so that there will be times with huge price spikes in the north. This will benefit Mercury and Genesis, Contact will probably be fairly neutral, and the loser will be Meridian.
Some of this disconnect in pricing can be recovered by the South Island generators purchasing FTRs (Financial Transmission Rights). These FTRs are sold via an auction process and there is no guarantee that any generator will get anywhere near the amount of rights they require.