Emissions Reduction Minister Shane Rattenbury told an ACT estimates hearing on Thursday it would be difficult to achieve the 2025 target of a 50 per cent drop in greenhouse gas emissions from 1990 levels.

"The short answer is that we're working hard to achieve it, but it will be hard to achieve the 2025 target," Mr Rattenbury said.

"The 2025 target is I think the most challenging target that we have in a sense.

"By the time we get to 2030, which is the next target, there would be a lot more technological change, but not as many by 2025."

The environmental targets laid out by the ACT government's climate change strategy committed the territory to a 40 per cent drop in emissions by 2020, 50 per cent by 2025 before a target of a 65 per cent drop by 2030.

An interim target of a 90 per cent drop in emissions by 2040 has been set before net zero emissions would be reached by 2045, according to the strategy.

Mr Rattenbury told estimates that while the ACT recorded a 45 per cent reduction in emissions by the end of last year, with the impact of COVID helping to improve those figures.

"Our view was the 45 per cent was an aberration in the sense it picked up the COVID period where people worked from home and there was not a lot of traffic on the roads," he said.

"We believe the true emissions reduction figure is around 40 per cent."

The minister said the 2020 target was largely reached due to the ACT's energy supply transitioning to being 100 per cent renewables.

Mr Rattenbury said emissions reduction targets could be more difficult to reach, with more people expected to be on Canberra's roads.

"There's no single, large source of emissions reduction in the same way the move to 100 per cent renewable energy cut out 40 per cent of emissions," he said.

"Population growth and a range of other factors will see a growth in transport emissions and putting pressure on the targets."

The possibility was also raised that some existing climate change policies might have to be scaled up or adjusted in order to meet the emissions reduction targets.

The Greens leader also conceded a legally mandated review of the ACT's climate change and emissions reduction policies was five months behind schedule.

A 10-year review into the policies was slated to be submitted to the Legislative Assembly by October 2020, but Mr Rattenbury said the report had yet to be commissioned.

"We need to get onto it. I believe it was due during the caretaker period," he said.

Legislation required the policies to be reviewed after five and 10 years.

The five-year review was handed down in 2015 but was done internally.

Opposition spokeswoman for environment and emissions reduction Leanne Castley said it was disappointing a decision had not yet been made on the 10-year review.

"These are laws Minister Rattenbury is responsible for," she said.

"Canberrans are keen to know whether these laws are working or not and, as importantly, how the laws are helping them and our environment."

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