Mr Rattenbury has been under pressure to explain why Unions ACT was allowed to run big wrap-around advertisements on buses promoting the tram line as a job creator, despite a ban on political advertising on buses.

On Wednesday, he tabled a "clarification" on the advertising rules, which says that the ban on political advertising only applies to "advertising of or by a registered political party" and "does not include issue-based campaigns".

On the face of it, this new wording would allow advertising by groups that have been set up to support the political parties, but Mr Rattenbury said the intention was clear.

Asked how the change in advertising policy came about, he said the advertisements had been approved by ACTION's advertising manager, Go Transit, without referral to anyone in government.

He became aware of the issue when Mr Peters called him on June 30 to alert him, saying something "along the lines of 'I thought I'd better have a conversation with you about how these [ads] sit with the existing guidelines'."

The bureaucracy had not provided written advice on the issue, he said.

Mr Rattenbury said he had told Mr Peters that in his view the advertisements did not breach the ban on political advertising but rather were issues-based. Now, he had provided written clarification.

If anti-tram group Can the Tram wanted to advertise on the buses, they, too, would be approved, Mr Rattenbury said.

"There is a very clear and easy difference between political parties advertising and people who are running an issues-based campaign," he said, insisting the tram ads were no different to Animals Australia's bus advertising campaign against live animal exports.

An attempt to build a "conspiracy theory" around the bus advertising was dishonest and did nobody any favours, Mr Rattenbury said.

The ACT Electoral Commission has separate rules about advertising on election issues. It requires any "electoral matter" to include an authorisation statement and defines electoral matter as any information intended to, or likely to affect, voting at an election, including information about an issue before voters at an election.

The definition clearly covers advertising about the tram and requires the advertisements to carry an authorisation statement, as they do.

Alex White, of Unions ACT, said he had not sought advice about whether the ads should be authorised. "We run political campaigns all the time. We know that when you talk about politics it has to be acknowledged."

He had not been aware of the ban on political advertising on buses, he said.

He would not say how much Unions ACT was spending on the bus ads and on the wider pro-tram campaign but said the ads would stop at the end of the year because next year strict spending caps would begin on political advertising in the lead-up to the election.