The state government is under pressure to sweeten the sale of a majority stake in the WestConnex toll road project after investment bank analysts warned that actual traffic volumes were unlikely to meet forecasts.

In a detailed report to large investors, Citi analysts have calculated that actual traffic on WestConnex in 2031 will be 10 per cent below forecasts made in the project's business case due to factors such as lower spending by households in Sydney's west and new public transport such as a metro railway offering alternatives.

The updated business case, released in late 2015, forecast average weekday traffic of almost 164,000 vehicles on the widened M4 motorway between Parramatta and Homebush, 132,400 on the so-called M4 East tunnels, and between 100,000 and 111,000 on the M4-M5 link between St Peters and Rozelle.

The Citi analysts said WestConnex was unlikely to deliver an "appropriate return" for a bidder such as Transurban unless better terms on the sale process were reached with the government.