The NSW Roads Minister has refused to answer questions about the planned Westconnex Rozelle interchange, despite her agency being responsible for fully funding and delivering it.

Melinda Pavey was peppered with questions about the interchange at a budget estimates hearing, but kept repeating it was a matter for her colleague and Westconnex Minister Stuart Ayres.

"Just ask Stuart — he's going to be here this afternoon," a visibly exasperated Ms Pavey said.

Upper House Labor MP Daniel Mookhey persisted with more questions.

"Is the Rozelle interchange part of WestConnex or not?" he asked.

"Could you please ask the relevant minister," Ms Pavey replied.

Mr Mookhey said it was unacceptable that Ms Pavey was unable to answer basic questions about the agreement, which resulted in more financial risks to her agency and to taxpayers.

The interchange is part of the third stage of the Westconnex but under Transurban's deal to buy 51 per cent of the motorway, Roads and Maritime Services agreed to carry the cost and risk of completing the interchange.

"Eight years into this government a roads minister in charge of a multi-billion-dollar infrastructure program can't even answer the most basic questions," Mr Mookhey said.

"It's embarrassing for the people of New South Wales."

At one stage during the hearing Ms Pavey gave Mr Mookhey a blank stare when he asked whether Roads and Maritime Services' $1.6 billion "State Works Contribution" to the third stage of WestConnex was included in the project's $16.8 billion price tag.

"What do you mean by state works?" Ms Pavey replied

"I'm actually asking what is your knowledge of the term 'state works contribution'?" Mr Mookhey said.

"No I'm asking you what state works is," Ms Pavey replied.

"Do you know?" Mr Mookhey said

"I'm asking you what you mean by state works?" Ms Pavey repeated.

The bamboozling exchange then continued.

"Any clue? Do you have any clue?" Mr Mookhey asked.

"Again Mr Mookhey I ask you to ask Stuart Ayres these questions this afternoon, he is the minister responsible for this." Ms Pavey said.

The Opposition's roads spokeswoman, Jodi McKay, said it was an embarrassing performance.

"I was gobsmacked watching the Roads Minister's performance today. She knows nothing about her portfolio," she said.

"What is the cost of WestConnex? The people of NSW want to know who is in control of this project."