The NSW Government has suffered a blow in its bid to compulsorily acquire properties for the final piece of the controversial WestConnex puzzle.

Property firm Desane took the Roads and Maritime Services to court over its plan to buy its head office in Rozelle.

The publicly-listed company claimed its office did not need to be acquired for stage three of the project.

Today the Supreme Court agreed.

Desane chairman John Sheehan said the victory followed a battle of biblical proportions.

"It was always our belief at the board level that there was no need to acquire our property," Professor Sheehan said.

"Desane is a relatively small public company and we have been [in] somewhat of a David-and-Goliath battle with the state of New South Wales."

Judge David Hammerschlag determined that the proposed acquisition notice was "invalid" as it was "given for an improper purpose."

Namely, that Roads and Maritime intended to purchase the land as part of a plan to build 10 hectares of parkland in Rozelle at the proposed M4-M5 interchange and not specifically to build the WestConnex tunnel.

It was therefore outside the reach of the department to purchase.

The decision by Judge Hammerschlag followed an 18-month legal battle.

"One of my [areas of] expertise is compulsory acquisition law and practice, so possibly they chose the wrong company," Professor Sheehan said.

"You simply can't as a Government, no matter what agency you are... you can't just simply say, 'there's a property over there, we just might acquire it'."

The company is expected to seek costs at a later date.