"It's nearly back to the beginning of the carbon price" in terms of the proportion of the market, Hugh Saddler, principal consultant with Pitt & Sherry, said.

Combined with a pick-up in black coal demand, the share of the NEM supplied by coal was 75.3 per cent, or the highest in 26 months.

Coal is largely replacing gas as more of that energy source is diverted to lucrative export markets, while hydro's share - at 7.8 per cent in May - continues to drop. Hydro generators had ramped up production to take advantage of the carbon price and now also face falling storage levels as drier conditions set in.

Meanwhile, wind generation has continued to grab market share despite new investment in wind farms all but frozen because of an impasse over the Renewable Energy Target that has been resolved only in the past month.

At an annualised rate, wind supplied about 5.5 per cent of the NEM in May, with generation in South Australia, Victoria, NSW and Tasmania topping gas-fired electricity for the first time, Pitt & Sherry said.