Members of the Australian Manufacturing Workers Union, the Community and Public Sector Union and the Australian Services Union say they have been refused entry to the stakeholder lock-up next Tuesday.

Only the ACTU has been allowed access to the lock-up and the organisation says its usual five seats have been reduced to three.

But a spokeswoman for the Treasurer said the stakeholder lock-up was two-thirds the size of last year's and the situation was out of their hands.

In a joint statement issued by the ASU, AMWU and CPSU, they said the move was unprecedented.

"Unions and community groups have always been welcomed to the budget stakeholder lock-up so that they might best understand how the budget affects their members and their industries," the statement says.

"To deny places to trade unions is beyond the pale and shows the true nature of this government. They are trying to escape any accountability for their budget."

ASU NSW and ACT secretary Sally McManus said every union that had applied to attend the budget, with the exception of the ACTU, had been denied entry.

"Even under the Howard government, you just had to be a member of a body representing the people and you got in. And this is an important part of our democracy," Ms McManus said.

Unions said the explanation given by the federal government was that only peak bodies would be allowed entry, and that the venue was at capacity and no places were available.

But Ms McManus said a lot of charities were attending the lock-up in addition to their peak body, the Australian Council of Social Service.

The spokeswoman for Treasurer Joe Hockey said this year's stakeholder lock-up was considerably smaller than in previous years, cut by about a third.

"The available room is smaller due to space restrictions in Treasury – this is out of our hands," she said.

"The ACTU is the peak body representing unions in Australia including the above four listed unions."

It is understood that this year's lock-up will take in more seniors groups and more welfare groups because of their concerns about changes in those areas in the budget, in addition to more disability groups.

CPSU national secretary Nadine Flood, who represents thousands of Canberra's public servants, said the move was a terrible look for the government.

"The staff numbers in Treasury might have shrunk but the meeting rooms haven't, so it's hard to see any real justification for excluding unions from the lock-up," she said.

"We represent hundreds of thousands of workers whose jobs and futures are on the line. Surely if big business, employer and industry groups are considered stakeholders, workers can be recognised as having a stake in the budget?"

AMWU national secretary Paul Bastian said his union had only requested one seat in the lock-up but was denied entry.

He said several of the National Commission of Audit recommendations would put 200,000 workers out of a job, including a number of the AMWU's members.

"This is an unprecedented attack on transparency," he said.

"I shudder to think about what the budget contains for working people if workers' representatives are being locked out.''

ACTU president Ged Kearney said she had never seen a budget lock-up where representative stakeholders had been purposefully locked out.

She said it was a blatant attempt to cut unions out of the first round of analysis of the budget.