Longer-dated futures also increased, with the 2022 futures price lifting from $100/MWh to $119/MWh at Ōtāhuhu and from $85/MWh to $101/MWh at Benmore.
In the following quarter, Mercury said electricity futures prices rallied after Rio Tinto announced that it planned to keep its majority-owned Tiwai Point aluminium smelter open until 2024.
Separately, Mercury chief executive Vince Hawksworth announced changes to his executive structure, with seven executive roles reporting to him.
The roles are the current positions of chief financial officer, chief marketing officer and general manager people and performance and newly created roles of general manager generation, general manager portfolio, general manager sustainability and general manager customer.
Disestablished executive roles are general manager retail and digital, general manager hydro/wholesale, general manager geothermal and safety, and general manager corporate affairs.
The confirmed executive roles are chief financial officer William Meek; chief marketing officer Julia Jack; general manager people and performance Marlene Strawson; and current general manager hydro/wholesale, Phil Gibson, who has been appointed general manager portfolio. The other roles are subject to a recruitment process.
Mercury's result for the December 31 half year will be released on February 23.
The company's shares last traded at $6.97, having gained 30 per cent over the past 12 months.