In recent months, the state government has been embarrassed by a string of leaked sensitive cabinet documents relating to cost blowouts of its major infrastructure projects.

But a document released under freedom-of-information laws has revealed it spent nearly $200,000 in one month investigating the source of leaks in a single week in July alone, mostly relating to cost blowouts of the controversial WestConnex Motorway.

But under questioning from the opposition in question time on Tuesday, Transport Minister Andrew Constance said there was a need to investigate "major security breaches" within his department because of rising global cyber and physical terrorism.

"If you think information that resides within the Department of Transport being leaked is some sort of laughing matter in today's world ??? in light of what's happening in the UK, in light of what's happening elsewhere around the world, you are kidding yourself," Mr Constance said. "We live in an era that relates to cyber terrorism and a whole raft of security issues.

"You only need to look around the world and what is going on. If it relates to the safety and security of the community, then I'm happy to stand here and debate [the matter]."

In the week in question, Labor obtained cabinet material estimating the Sydney gateway project connecting the WestConnex toll road to the airport would more than double in cost to up to $1.8 billion and that a compensation bill associated with the project could rise as high as $1 billion.

Mr Foley rejoined: "Is [the Minister] seriously suggesting that he suspects public servants in his own department of engaging in preparations for terrorist activity?

"If you want to play the terrorism card, bring some evidence, mate."

Mr Constance suggested that Opposition Leader Luke Foley had "aided and abetted a crime" and called on him to report the source of the leaks to the police.

"Whoever it is, and I'm sure he's made the commitment of a very, very good promotion in the future, we are right to investigate," he said.

At a budget estimates hearing earlier this month, the Secretary of Transport for NSW, Tim Reardon, confirmed the department had referred a recent string of leaks to both the police and the Independent Commission Against Corruption.

In the September inquiry, Mr Constance declined to provide estimates of the cost of other transport projects such as the revamp of Central Station and the Newcastle and Parramatta light rail lines, declaring such estimates "commercial in confidence".

The Greens' transport spokeswoman, Mehreen Faruqi, accused the government of shrouding information about major projects in secrecy, saying "it's no wonder people are leaking".

In the same July week that was the focus of the department's $200,000 investigation, a cabinet document handed to Labor also revealed that the increasing use of the M5 cashback scheme could blow a $90 million hole in the government's budget.

But the government has also been hit by other damaging leaks.

Fairfax Media has also reported on leaked documents revealing the extraordinary $18 billion cost of a planned F6 motorway extension, and that planners on that project had been told to disregard public transport alternatives to the motorway.

Other leaks have been less impactful. Last July, Labor touted yet another leaked document from within the department it claimed showed that pensioners will have to wait for five to 10 days before claiming their discounted Gold Opal cards.