The State Parliament will be denied a chance to investigate the biggest motorway in the country's history after a Shooters and Fishers Party MP cut a deal to save a park near his inner west home.

Robert Borsak said he had agreed to vote against an inquiry into the $13 billion WestConnex after Roads Minister Duncan Gay committed to preserve Ashfield Park, which had been threatened by the motorway.

Mr Borsak said he had been asked by Mr Gay three weeks ago to block an inquiry into the WestConnex motorway. He said the agreement helped save the park and nearby Yasmar Estate.

''That's effectively what's happened, yes,'' Mr Borsak said.

Mr Borsak lives in Ashfield and had been a strong critic of the WestConnex and its potential traffic impact on the area.