But Chief Minister Katy Gallagher says she will draft legislation in 2013 to change the ACT's self-government Act to allow for the addition.

The opposition is likely to support the move but says the cost would be more easily justified if the government had not given Greens Minister Shane Rattenbury a "$1 million" staffing allowance for his office.

Ms Gallagher made it clear after the October election that the Assembly workload was becoming less manageable for five ministers.

In an end of year interview with The Canberra Times, the Chief Minister said she hoped to take on an extra minister in the new year and that could be done without approval from the federal government. "I've got it on the legislation program, we actually have to amend the self-government act and it has to go through that process," she said.

"That will lengthen the time [it takes].

"It's something I said I'd like to do within the first year but, as with all things, it doesn't come easily."

Ms Gallagher said the government would have to consider several factors before increasing the ministry, including the logistical issue of where a new minister's office would actually fit in the Legislative Assembly building in Civic, as well as the cost to the ACT.

"It would probably come at about an extra million dollars," she said.

"I'm not sure of that figure in total, you'd have the minister's salary - about $170,000 - and then a staffing allocation of about $500,000 and then associated on-costs with that.

"So it's another balancing-up act. Is that what we can afford?"

The government would also have to consider whether it could afford to promote a backbencher to the ministry, which would mean it would have fewer MLAs who could sit on the Assembly's committees. But the Chief Minister said that if the number of ministers was not increased the workload would become unmanageable by the time the Assembly reached its term in 2016.

A panel is also assessing whether the number of MLAs should be increased from 17 to 21 or 25.

"I think by the time the city's 400,000 and you're dealing with the issues of a place of that size it's going to get too much," Ms Gallagher said.

"I'm trying to avoid the point where governance and government actually suffers."

Canberra Liberals leader Zed Seselja said the party had never objected to increasing the size of the government cabinet.

"The cost would be more easily justified if the Green/Labor government hadn't given the Greens Minister a $1 million staffing allocation," he said.

In November, The Canberra Times reported that that Greens minister Shane Rattenbury had been given a $790,000 staffing allowance, which was more than both the Deputy Chief Minister and the Opposition Leader.