Tomago Aluminium, a joint venture between global companies Rio Tinto Alcan, Gove Aluminium Finance Ltd (CSR and AMP) and Hydro Aluminium, has signed an 11-year base-load power supply contract with Macquarie Generation, securing the company's power supply until 2028.

The breakthrough represents the largest energy deal ever signed in New South Wales and gives Tomago Aluminium critical certainty in terms of its long-term investment, and clarity on the business challenges ahead, the company said.

The agreement will replace the existing power supply contract with Macquarie Generation when it expires in 2017.

Tomago Aluminium also said the deal opened the door for continued discussions with Bahrain-based Midal Cables International (Midal), which is looking to enter into a long-term hot metal supply opportunity with Tomago Aluminium and build a $30 million aluminium conductor and rod factory on land adjacent to Tomago Aluminium's operations.

Given recent uncertainty surrounding Tomago Aluminium's long-term power supply arrangements, discussions with Midal were reluctantly suspended in September.

The Midal arrangement alone would mean the creation of 290 jobs, most of them in the Hunter region.

"This is an excellent outcome for the long-term future of Tomago Aluminium and the 5,300 jobs we sustain directly and indirectly," Tomago chief executive Brian Cooper said in a statement.

"It's significant in terms of the Hunter Region, New South Wales and national investment and employment."

"Tomago Aluminium and Macquarie Generations evolution and development have been intertwined for the life of the smelter, and the long relationship we have in place will be important as we face the challenges of climate change together," Mr Cooper said.