Genex shares surged as much nearly 38% in early trade Monday to $AU0.255, a more than three-year high. The broader S&P/ASX200 was up 0.2%.
The AU$0.275 per share offer represents a 48.6% premium on its last close from Friday.
The Japanese firm, which owns a 7.7% stake in Genex, had previously made an offer to buy the company at a price of AU$0.240 per share, Genex revealed in a regulatory filing on Monday.
An independent committee of the board of directors of Genex had determined the earlier offer undervalued the Australian firm.
Genex said the committee has unanimously determined the new offer is in the best interests of its shareholders.
Outbound Japanese buyout activity has made its strongest start to the year so far in 2024 for six years, according to LSEG data.
J-Power is a joint owner and developer of Genex's Kidston wind project and Bulli Creek clean energy project, which are both in the Australian state of Queensland.
The new takeover bid requires 75% support from Genex shareholders and approval from Australia's Foreign Investment Review Board (FIRB).
If that level of support is not reached, J-Power said it would launch an off market takeover at AU$0.27 per share which would need at least 50.1% of shareholder backing to go ahead.
Genex's major shareholder is Atlassian co-founder Scott Farquhar's Skip Capital, which holds 19.99% of the renewables firm.
Farquhar in 2022 led a consortium that made an indicative bid for Genex at AU$0.25 per share before walking away from the deal.
Farquhar and Skip Capital did not immediately respond to requests for comment from Reuters.
The move comes as part of the Japanese company's efforts to boost its renewable energy assets, including overseas, by 1.5 gigawatts by end-March 2026 from end-March 2018.
The company held almost 10 GW of renewable energy in operation worldwide as of April 2023, according to its website.
J-Power, which is Japan's major power generator with many coal-fired power plants and hydroelectric facilities, raised its 2030 emission reduction target last year by 1.3 million tonnes from 2013 levels.
Under the new target, J-Power aims to cut its CO2 emissions by 46% by 2030 from 2013 levels of 48.77 million tonnes, against an earlier target of 44%. The latest goal is in line with Japan's national target.
