The number of National staffers and MPs allowed in Opposition's pre-Budget lockup has been slashed, which "significantly constrains" the party's ability to provide a response to the Budget.
The party's finance spokeswoman, Amy Adams, said the move looked to be "a petty and vindictive attempt by the Government to limit our already limited ability to review the Budget documents".
But a spokesperson for Robertson said the refurbishment of the usual lockup room means there is less room for Opposition MPs and staffers.
No other rooms were available at the time as select committees were being held, the spokesperson said.
Although the select committee schedule shows the last meeting on Thursday finishes just after midday, the spokesperson said it takes time for the room to be made secure.
As well as this, because Jami-Lee Ross was now an Independent Opposition MP, he is entitled to a place in the room with one staffer.
The Act Party has also requested two spots in the lockup. Last year Act did not attend.
The four Act and Ross representatives, as well as the smaller room, means there is only eight spots for National staff and MPs.
Last year, National had 16 representatives in the lockup room.
Every year, media and analysts get three and a half hours with the Budget before it is released under lockup conditions.
The Opposition, however, is given just an hour to review the documents before the embargo lifts at 2pm on Thursday afternoon.
But an email to National from the office of the Minister of Finance revealed that this year, National were allowed just eight people.
"Sadly, due to room seating constraints we will need you to nominate the eight people that you wish to attend the Opposition Budget lock-up," an official from Robertson's office said.
Adams said: "We are told by the Government this is an all-new approach to a Budget that hasn't been seen before – that actually puts more pressure on us to be able to absorb it."
The decision "significantly constrains our ability to go through the Budget documents," Adams said.
Asked why the decision was made, a spokesperson for the Treasury said the Opposition's lock-up, and any decisions around it, were run by the Minister of Finance's office, not the Treasury.
According to the emails, the decision to reduce the number of National people in the lockup was made on Monday.
Yesterday, National released information that Robertson confirmed was Budget sensitive.
Late last night, the Treasury said it had gathered sufficient evidence to indicate that its systems have been deliberately and systematically hacked.
Robertson said he had asked National to stop releasing the information – "given the Treasury said they have sufficient evidence that indicates the material is a result of a systematic hack".