The Turnbull government's signature energy plan fails to anticipate sudden exits of ailing coal-fired power plants and could lock in targets for a decade - two key design flaws that incline the ACT government to block it, Shane Rattenbury, the territory's energy minister said.

"It's very difficult for us to sign up [to the National Energy Guarantee] in the current form," Mr Rattenbury told Fairfax Media. His support will be crucial for the NEG to succeed because it requires all states and territories to sign up. The Federal Government hopes the NEG will reduce power prices, cut carbon emissions and improve reliability of the grid.

ACT Energy Minister Shane Rattenbury won't back the National Energy Guarantee in its current form. Credit:Sitthixay Ditthavong

Mr Rattenbury said the plan's weak 2030 emissions target for the electricity sector - which may actually be achieved as early as 2020 as renewable energy surges - was another issue Josh Frydenberg, the environment and energy minister, had shown no sign of budging on.

There had been "absolutely zero movement" from the goal of cutting sector emissions 26 per cent from 2005 levels by 2030, the minister in the Labor-Greens government said. That target is at the low end of Australia's 26-28 per cent pledge in the Paris climate agreement, and most analysts agree pollution cuts could go much deeper.