The WestConnex Delivery Authority plans to build the interchange on a former landfill site, but has not reached agreement with that site's former owner over the land valuation.

"Our view is the government has grossly undervalued the site," said Christopher Biggs, the group general manager and company solicitor for Dial A Dump Industries Group, the site's former owner.

The WestConnex Delivery Authority compulsorily acquired the site in December, and on Wednesday revealed plans for the multi-lane interchange to be built there in the next four years.

But the two having failed to reach terms on the 16 hectare facility, Dial A Dump recently filed an objection in the Land and Environment Court.

"The parties are, shall we say, some distance apart," said Mr Biggs, though he would not be drawn on how far.

The dispute over the bill for the site should not slow down construction of the motorway, as the government is already in possession of the land.

But Mr Biggs said the price offered did not take into account "any compensation for the loss of business" and the fact that about 100 employees had to be relocated.

"This was an enormous business there," Mr Biggs said. He said the company had run landfill, waste collection, recycling and waste transfer businesses from the facility.

"We have been for all that time without a home for our business and without compensation either," he said.

A spokeswoman for the  delivery authority said the NSW Valuer-General had completed a valuation of the property "on July 30, 2015, after the landowners provided documentation required to support the valuation process".

"As per the owners' right, an objection has been made to the Land and Environment Court," the spokeswoman said. "As the matter is before the court, WDA is unable to comment further."

In Parliament this week, the chief executive of the Sydney Motorway Corporation, Dennis Cliche, outlined the components of a $500 million cost increase of the total WestConnex project, from $14.9 billion to $15.4 billion.

Mr Cliche said about $216 million was for stub tunnels to connect to an eventual extension south to the Sutherland Shire, and about $300 million was for "additional scoping works around the St Peters interchange".

Mr Cliche said about half that $300 million was for land acquisition.

On Wednesday, Roads Minister Duncan Gay revealed the scope of the motorway interchange and its connections to Euston Road, Gardiners Road and Campbell Street. The plans show that about half the former Dial A Dump site would eventually become open space or parkland.

Mr Gay rejected criticism that the interchange could increase traffic in the area.

"There are a lot of people that need to get in and out of the area of Alexandria, Erskineville, Mascot, through there," he told ABC Radio on Thursday.

"Because if you have a look down there, Sydney City Council has developed a large number of units with people living there and those people need to get out to get south."

Sydney Lord Mayor Clover Moore called the interchange a "shockingly disruptive proposal".

"Within a decade it will be dumping more than 30,000 cars a day in the already gridlocked roads of Erskineville, Alexandria and Green Square," Cr Moore said.