Being in the index could see Meridian drawn to the attention of a new class of international investor.

Meridian Energy, New Zealand's largest electricity generator, has been added to a long list of the world's most sustainable companies.

On September 21 the Wellington-headquartered company becomes only the second New Zealand company to be included in the Dow Jones Sustainability Indices (DJSI), joining Auckland International Airport.

Launched in 1999, the index is widely regarded as the most credible international sustainability index. Drawn from a group of large publicly listed companies, the 10 largest NZX-50 companies were invited to apply to join the Asia Pacific index.

Being added to the index could see Meridian, which was partially sold by the Government in 2013, drawn to the attention of a new class of international investor.

"The purpose of it is to benchmark yourself against like-minded companies internationally, so that a certain investor class, particularly those looking at putting their money into sustainably businesses can have a filter...for where they want to put their money," Meridian spokesman Guy Waipara said.

Administrators of the index wanted detailed information about Meridian's business, Waipara said, well beyond the core business of electricity generation, requesting information about the success of community programmes, waste management and supply chain and procurement.

"Joining the DJSI provides our customers, shareholders and communities confidence that Meridian operates responsibly in our aim to be a high performing and resilient business."

The first time Meridian, which has a board subcommittee focused on sustainability, had been invited to join the index.

Meridian generates around a third of New Zealand's electricity, mostly from large South Island hydroelectricity schemes. It also has wind farms in Wellington, Waikato, Manawatu, Southland, Australia and Antarctica.

The government retains a 51 per cent stake in Meridian.

The DJSI extends well beyond renewable energy companies, with other companies in the Asia Pacific index including Japanese tyre-maker Bridgestone, Korean mobile phone maker Samsung and the Commonwealth Bank of Australia.