Meridian - the primary supplier to Tiwai - has fallen more than 7.75 per cent ($680m) to $5.01. Mercury dropped by 22c or 4 per cent to $5.23. Contact Energy and Genesis Energy also opened weaker.
But the prospect of surplus power supply and falling prices if Tiwai closes has also seen Mercury, Contact and Genesis fall by about five per cent.
Energy Minister Megan Woods has warned the owners of the Tiwai aluminium smelter not to expect taxpayer funds to help it remain open.
The owners of the aluminium smelter said on Wednesday that there were seeking talks with the Government amid a strategic review which could see the operation closed.
Rio Tinto said it would "will conduct a strategic review of its interest in New Zealand's Aluminium Smelter [NZAS] at Tiwai Point, to determine the operation's ongoing viability and competitive position".
This would include talks with the Government and electricity suppliers.
Meridian Energy, which supplies electricity to the smelter, said the review options included closure.
But in a statement, a spokesman for Woods described the review as "a commercial process by a commercial operator" and signalled there should not be an expectation of a bail out.
"We hope that these discussions and work by NZAS and Rio Tinto result in the smelter remaining open and continuing to back the Southland economy by trading on its position as producing the world's greenest, low-carbon aluminium," Woods said.
"The New Zealand Government has had a clear position since 2013 under the Key/English Government that there will be no more financial assistance from taxpayers for Rio Tinto, which is already supported by Meridian for the power it uses. This hasn't changed."
Opened in 1971, the smelter is New Zealand's largest electricity user, consuming around 14 per cent of total generation, mainly from the nearby Manapōuri Power Station.
The smelter says that, including contractors, it employs around 1000 people.
Rio Tinto said "under current market conditions and with high energy costs, we expect the short to medium outlook for the aluminium industry to be challenging and this asset to continue to be unprofitable.
"Rio Tinto intends to hold discussions with the Government of New Zealand and energy providers to explore options and identify economically viable solutions to find a pathway to profitability for the asset."
Meridian Energy first announced the review in a notice to shareholders this morning.
"The review will consider all options for the future of the smelter, including the option of closure," Meridian said in a statement.
"Rio Tinto has advised Meridian that it will provide the market with an update on the strategic review by the end of the first quarter in 2020."
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Comments
Watch our power prices go up yet again.
How many years has this multi-national and it's wealthy share holders been subsidised by tax payers and electricity consumers, both commercial and residential?
2013 may have been the first time they got a hand out, but it's not the first time they got cheaper power than was commercially viable. Right back when the plant was built, the government built a damn to support it, in return for the contribution the smelter would make to the country. It's time everything was laid bare for kiwis to see, including a sound understanding of costs and returns.
If our consumption dropped by 14% over night, we would meet an obligation under the Paris agreement, get cheaper power to consumers and have the capacity to accommodate the growing electric vehicle fleet in one go. The fact that gentailer shareholders loose if the smelter goes, highlights why the power companies should never have been commercialised in the first place. The goals of a commercial operator are short term profit, whereas the goals of a nation are a stable and clean power supply for the long term.
Power generators make massive profits as their cost of inputs is virtually nothing- 'free' rain. The physical dams were all paid for by our tax dollars and these assets sold, and resold, and resold. These companies' "value" is the goodwill that the purchasers paid over book price- so a whack to those values does not make me cry. They have pillaged NZers with expensive electricity for years and earned super-normal profits. An excess of electricity should reduce South Islanders' power bills- that is something to cheer about. The whole Tiwai smelter was a jobs project as NZ does not mine bauxite- the raw material of aluminium- we were just the mugs that offered a subsidy to Alcoa.