Toll road giant Transurban has told a New South Wales parliamentary inquiry it is not sure how much profit it will make from Northconnex, the $3 billion tunnel it is building in Sydney.

Transurban owns 13 of the nation's tolled roads, in Sydney, Melbourne and Brisbane.

It is building Northconnex, a nine-kilometre tunnel linking the M1 at Wahroonga to the Hills M2 Motorway at Pennant Hills, set to be completed by 2019.

The company is the first to give evidence at the inquiry looking at how tolls are set.

Northconnex is expected to cost $3 billion; around $800 million is coming from the federal and state governments, while Transurban along with two other companies are delivering the remainder — more than $2 billion.

In return, Transurban has struck a deal with the New South Wales Government that would allow it to receive:

Tolls from Northconnex until 2048

Tolls from Northconnex until 2048 An extension of tolls on the Westlink M7 until 2048 (it was due to expire in 2037)

An extension of tolls on the Westlink M7 until 2048 (it was due to expire in 2037) An increase in truck tolls on the Westlink M7.

Recouping cost to take decades

Labor MP John Graham asked Transurban's general manager Andrew Head how much Transurban would make from the deal.

"I haven't calculated exactly how much the tolls will be over that period of time," Mr Head said.

"We have to invest a very large sum of money today, and most of the returns that come through to recoup that capital investment come through over a 40-year period."

"So it takes a very long time to get back the original capital investment, let alone make a return on it. We're talking a couple of decades."

Mr Head told the inquiry about the many benefits he said Sydneysiders were enjoying because of roads built in the past 25 years.

"Life is significantly easier for Sydneysiders as a consequence of the roads that have been delivered," he said.

"[a total of] 155,000 hours of time is saved every single day by Sydneysiders that use the motorway network."