The west gate tunnel was just east west link stage 2 under a new name, and in all honesty should've been built first. It makes the east-west link business case stack up better, which is why Andrews built it.
The West Gate Tunnel is vastly inferior to what was originally proposed as the western half of the East-West Link (and the WestLink project before that). The western half of the EWL project was originally proposed by the Brumby Government in the late 2000s following the Eddington report (East-West Needs Assessment).
Brumby chose to prioritise the western part of the project between the Ring Road and Footscray Road and had been progressing the planning of the project until he lost power to Baillieu in 2010. When Baillieu came to power nothing much happened for a few years and there were very few major infrastructure projects underway in Victoria. Then when Baillieu was replaced by Napthine, it was identified that major infrastructure projects were required in order to hold onto power. The EWL project was quickly resurrected, but this time the eastern half between the Eastern Freeway and CityLink was prioritised. The western half was relegated to be 'Stage 2'.
When Andrews won the election in 2014 and cancelled the EWL project and released various versions of the Business Case with low BCRs, it was only the eastern half of the project that these versions of the Business Case referred to. The western half always had a higher business case than the eastern half.
The reason why the original WestLink project was vastly superior to the West Gate Tunnel is that west of the Maribyrnong River it followed a completely different alignment through West Footscray, Tottenham, Brooklyn and Sunshine West and provided new interchanges to provide access to existing underdeveloped areas in that region (the route is still shown on the maps available within Melway Online
). By contrast the West Gate Tunnel Project only widens the existing West Gate Freeway, providing no alternative access routes other than new connections to the Port of Melbourne area. It also doesn't provide any alternatives in the event of significant congestion or incidents and utilises a large section of the existing Ring Road in order to access the Western Freeway.
The other issue with the West Gate Tunnel Project is that it was an industry led project dreamt up by a private consortium with the aim of maximising their profits, rather than something that was properly planned by Government to best meet the needs of the community.