Dry weather and discounted power to NZ Aluminium Smelters' plant at Tiwai Point may weigh on Meridian's earnings over the second half, chief executive Neal Barclay said.
Meridian Energy said its net profit gained 19 per cent to $315m in the first half thanks to positive changes in the value of its hedging arrangements.
At an operating level, its earnings fell by nine per cent to $422m due to lower hydro generation in New Zealand and lower market prices in Australia.
That Ebitdaf number was still its second highest interim performance on record, following as it did from a record six months to December 2019, which was boosted by very high levels of hydro generation.
Meridian left its interim dividend unchanged at 5.7 cents and said it was looking at setting up dividend reinvestment scheme.
Chief executive Neal Barclay said that given constrained hydro conditions, and the "unique" Covid-affected environment over the half, that he was happy with the performance.
It looked like the weather would again play a big part in the company's earnings over the second half, he said.
"What we know at the moment is that national storage is around 72 per cent of normal, with a dry outlook in line with the La Nina weather pattern, so we are taking a cautious approach," he told the Herald.
In January, Rio Tinto, the majority owner of Meridian's biggest customer, NZ Aluminium Smelters (NZAS), announced that it had put off closure of the Tiwai Point plant from August this year to 2024.
Barclay said the new deal to supply power to the smelter was structured to be "significantly less" than the previous existing contract price.
"It gave them four years to work out how to close the plant down in a seemly fashion and give the Southland community time to evolve away from reliance on the smelter," he said.
"It will also impact the financial earnings of the business for the second half," he said.
Terms of the deal were confidential. "But it is a reasonable amount of money."
Chief financial officer Mike Roan said on a conference call that if the smelter had closed as planned in August, it was likely to have prompted at $690m to $1.3b reduction in the value of Meridian's generation facilities.
Along with its result, Meridian said it would go ahead with a new $395m the Harapaki Wind Farm farm - the country's second biggest - in Hawke's Bay, which was put on hold while the company prepared for the possible closure of Tiwai.
Barclay said that several more projects the size of Harapaki would be needed if New Zealand was to achieve its carbon emission ambitions.
"We need to gear our business up to build at least one Harapaki-sized project every three years, for the next 30 years," he said.
"It's an exciting opportunity and we need to look at balance sheet flexibility to enable us to do that."
In late December 2020, the Electricity Authority released a final decision confirming an undesirable trading situation - spilling water when rainfall was high - in December 2019 while wholesale prices remained abnormally high.
The authority found a "confluence of factors" had led to a highly unusual period of wholesale market operation.
The Authority is intending to publish a preliminary "actions to correct" paper for consultation in late March 2021.
Barclay said the incident was never intended to maximise profits by boosting the wholesale power price by spilling water, saying it was to manage a flood event.
He accepted that up to 28 gigawatt hours of water was wasted in the process and that Meridian had adjusted its systems to ensure there was no repeat.
Meridian is still looking at building a 100 megawatt battery in the North Island, which it says would effectively increase capacity over the Cook Strait cable. A North Island battery would take four years to build.
Looking ahead, It was likely that New Zealand would need a relatively small amount of gas and/or coal generation for at least another decade.
"In short, the industry needs to migrate away from fossil-fuel-based generation while still relying on these in very dry years," Barclays said.
Mercury, New Zealand's second biggest by market cap, is just over 51 per cent owned by the New Zealand Government.
US investment giant BlackRock has seven per cent after its renewables-focused exchange-traded funds (ETF) picked up stock last year and early this year.
Meridian's shares last traded at $5.58, down 3 cents, and compares with $9.40 at the peak of the ETF buying spree early this year.