"This will help support electricity generation through winter and ensure electricity and gas supply to our customers and other market participants", Genesis chief executive Marc England said.
As part of the deal, Methanex will idle one of its Motunui facilities for around three months. At the start of April it said it would close another one of its plants permanently, with the loss of 75 jobs.
A spokeswoman for Methanex said Friday's announcement would result in no job losses.
"Methanex plays an important role by underpinning the long-term development of New Zealand's upstream gas sector," Methanex New Zealand managing director Dean Richardson said.
"We are pleased to have reached this collaborative agreement with Genesis to demonstrate and support the country's energy stability and security."
The deal was hailed by the industry which has warned that current conditions in the energy market are putting jobs at risk.
"[Methanex] is making a notable contribution to New Zealand by foregoing gas that it has a contract entitlement to use in its own business," Gas Industry Company chief executive Andrew Knight said.
In recent weeks the Gas Industry Company has begun consultation about the future for the gas market which warned that as soon as 2026 there may not be enough gas to support electricity generation.
Currently the lakes which fuel New Zealand's hydroelectricity stations are at unusually low levels, while production from Pohukura, New Zealand's largest producing gas field, has been dropping sharply.
Enerlytica analyst John Kidd said the deal would remove some of the fuel stress in the electricity market, which was currently pricing the risk of supply shortages.
"It's not going to make the risk go away, and I'd still expect a solid risk premium to be built into near term pricing, and in fact it's being built into long term pricing as well."
Major electricity users have been warning the current wholesale prices are putting thousands of jobs at risk.
Already the Whakatane Mill has announced plans to close, while in nearby Kawerau Norske Skog is consulting with workers about a possible closure of its paper mill.
Other major industrial users are understood to have lobbied the government directly for an intervention in the energy market to reduce prices.