A union campaign spruiking the benefits of light rail was approved on Canberra buses despite a ban on political advertising and anti-pokie material being blocked shortly before the 2012 election.
The Unions ACT advertising was wrapped around four buses as part of a larger campaign to highlight the jobs created by light rail.
Activist group GetUp! was unable to run an anti-poker machine campaign on Canberra buses in 2012 because it was deemed too contentious and political.
The organisation had hoped to raise awareness of problem gambling in the ACT and pay for advertisements on buses driving around Parliament House.
The campaign called for $1 maximum bets and criticised supermarket giants Coles and Woolworths for their involvement with poker machines.
Shadow transport spokesman Alistair Coe said it appeared the government had favoured advertisements that supported its policies.
"It seems there are two sets of rules; one for those pushing the government's message and another for everyone else," he said.
"There has to be a common set of rules that is enforced and they cannot just get changed on the day."
Territory and Municipal Services Minister Shane Rattenbury said the government was not responsible for approving advertising on public transport.
"There is a company engaged on behalf of ACTION to manage advertising on ACTION buses," he said.
"This company is responsible for the interpretation of ACTION's advertising policies and therefore determines which are ads approved.
"This company has changed since 2012 and there might have been a different interpretation of the policy."
Political advertising on public transport is banned although the ACT government has clarified the definition as material circulated by a registered political party.
"This does not include issues-based advertising which may have a political context, for example by a campaign group," Mr Rattenbury said.
When the campaign was launched in early July, Unions ACT secretary Alex White said advertising would be installed in shopping centres with postcards distributed across the territory.
"Unions ACT is launching this campaign to support the major jobs boost that light rail will bring, both in the short-term, and through the transformation of our local economy," he said.
"This is the largest public awareness campaign Unions ACT has ever run in Canberra."
Mr Coe said he did not support a total ban on political advertising on buses but there needed to be clearer rules.
Advertising in Canberra buses generated $545,000 of revenue for the ACT government in 2014-15 with another $18 million claimed from MyWay fares.
In September, the government tightened advertising restrictions by banning material promoting junk food, fossil fuels, gambling, alcohol and weapons.
Mr Rattenbury said changes would also provide more clarity for the company managing bus advertisements, with a year until the next territory election.
"Given that the policy guidelines are subject to the interpretation of the advertising contractor … we are also clarifying that 'political advertising' means political party advertising," he said.
"This does not include issues-based advertising which may have a political context."
The advertising restrictions have proved controversial with industry groups and NSW senator David Leyonhjelm described it as an unnecessary "nanny state" policy.
Mr Rattenbury rejected the nanny state assertion and said it was reasonable to consider what advertising should be displayed on government-funded buses.
"This is about being a socially responsible government that does not promote activities or items that have a negative impact on us as a society or as individuals," he said.
"We already prohibit tobacco and other items that have a detrimental impact on our society, this is simply an extension of this policy to reflect current issues such as obesity and problem gambling."