Government officials have confirmed that an area of about 11 metres in breadth will be taken off a one-kilometre stretch of St Peters' Sydney Park to make way for a feeder road for the motorway.

Roads Minister Duncan Gay declined to answer when asked if this was inconsistent with his previous statements to parliament about whether the park would be reduced by the six-lane WestConnex project.

When asked if Sydney Park would be retained in its entirety Mr Gay told parliament in November: "The short answer is yes."

It is understood the government does not consider the eastern strip of green land adjacent to Euston Road a part of the park.

The state government owns the land and says it has long reserved it for a road corridor.

The former site of a brick works, Sydney Park covers about 45 hectares and was used until the 1970s as a rubbish dump before being built over.

"Green space in the inner city is precious and it shouldn't be contracting," said ALP councillor Linda Scott.

Lord Mayor Clover Moore labelled the decrease in park space "unacceptable".

She has further criticised government for not releasing analysis of the traffic impacts of the major motorway before it signs contracts for building the road.

The Lord Mayor says she is concerned traffic from the motorway will be disgorged onto already-clogged inner-city streets.

"Based on the information we've seen so far, WestConnex looks like a tunnel to a traffic jam," she said.

A spokesman for the WestConnex Authority said that while "detailed" modelling was still under way the government had completed a "strategic" traffic review. Both will be released by late next year, the spokesman  said.

The City has resolved to commission its own traffic analysis in a bid to pressure to the state government.

The motorway is scheduled for completion in 2019.