NSW Environment Minister Penny Sharpe has defended a decision to omit the state's medium-term goal for emission cuts in measures introduced to parliament in October.

If passed, the bill will set into law the state's 2050 net-zero emissions target and establish an independent commission to review and report annually on the government's progress.

It will also enshrine a 2030 goal of a minimum 50 per cent cut in greenhouse gas emissions on 2005 levels.

Ms Sharpe said another target, for a 70 per cent reduction in emissions by 2035, had already been set in separate legislation.

"I want to make it very clear that we are not repealing that regulation," she told a budget estimates hearing on Thursday.

But the minister was unable to provide an update on how long coal-fired power station Eraring would stay open or how much it would cost the state's taxpayers.

She said keeping Australia's largest power plant open was "totally undesirable" but an independent commission would help mitigate its long-term effects on the climate.

"(The commission) will put advice to the parliament and to government about what are the choices that we have to make," Ms Sharpe said.

"If we are extending Eraring there's an impact and where are we going to find the other emissions reductions that we have to do?

"It's not easy but that's why (the commission) is so important."

Official modelling shows NSW is on track to cut emissions by 55 per cent on 2005 levels by 2030, and 70 per cent by 2035 under current policies.

Ms Sharpe said she thought the state was on track to meet its 2030 target of a 50 per cent reduction, but more work was needed.

"We've only reduced by about 18 per cent from 2005 (emissions), as we sit here today," she said.

"This is a big task and we need to make sure that we keep on with it."

Australian Associated Press