ACT Greens minister Shane Rattenbury says the government's ban on lightweight plastic bags should be tightened to make sure all bags used in Canberra are compostable.
And the opposition has questioned a government survey showing that 70 per cent of Canberrans want the ban to continue.
The government's interim review of the plastic shopping bag ban, released on Thursday, found that 58 per cent of 600 households surveyed supported the policy and 70 per cent wanted it to continue.
As well, 84 per cent of respondents said they now carried a reusable bag to the supermarket.
But the report said it was still too early to be certain the ban had reduced the amount of plastic bags going into landfill and noted other studies had found reusable bags needed to be used 11 to 14 times for the overall environmental impact of shopping bags to be reduced.
Canberra Liberals environment spokesman Brendan Smyth said the government was overstating the public support for the ban.
The Canberra Liberals went to the election in October promising to reintroduce free plastic shopping bags if they won government.
''Instead of a 600-person survey, we'll talk to the whole community and find out what they want,'' Mr Smyth said.
''It's interesting the way they've selected people to participate - I still get a lot of people saying bring back the plastic bag.''
Mr Smyth said the report also showed that the ''jury was still out'' on the effectiveness of the ban.
''It's very hard to see a net gain for the environment in this and that's been our position from the start.
''The government can't put any detail forward showing it's been successful.''
But Mr Rattenbury said the survey showed the ban had the ''overwhelming support'' of Canberrans.
The balance-of-power MLA said the government's next step should be to ban any shopping bags that were not completely compostable.
Mr Rattenbury said the interim report showed 94 per cent of Canberrans wanted plastic shopping bags to be completely biodegradable.
''Unfortunately, one unintended and perverse outcome of the ban, as it is currently legislated, is the provision of non-compostable and thicker plastic bags as a replacement to the banned lightweight plastic type,'' he said.
''Many of the bags that are called 'degradable' simply break into a thousand plastic pieces and never actually biodegrade. All in all, these substitutes run the risk of defeating the purpose of the plastic bag ban.''
The Canberra Times interviewed shoppers at Kambah Village yesterday and most said they were in favour of the ban. Tuggeranong resident Lisa Charles said she had found the policy ''a pain'' at first but was now happy to be using less plastic.
''The ban doesn't bother me,'' she said. ''I always take my own now and on the odd occasion that I forget, I buy a couple.''