Carbon emissions from the country's main electricity grid have risen since the end of the carbon tax by the largest amount in nearly eight years.

Data from the National Electricity Market, which covers about 80 per cent of Australia's population, shows that emissions from the sector rose by about 1 million tonnes, or 0.8 per cent, at an annualised rate last month compared with June.

Dark clouds: Carbon emissions from the power sector are back on the way up. Credit:Jonathan Carroll

That is the biggest two-month increase since the end of 2006, and came as a result of an increase in overall demand and a rise in the share of coal-fired power in the market, according to Pitt & Sherry's monthly Cedex emissions index.

"It is highly likely that the trend directions of electricity demand, generation and emissions seen in the last two months will become set in place," the consultancy said, adding that the emissions intensity of the power industry was rising after six years of falls.