Need to start looking at this in the context of the current political climate, there are a few things at play here.
1) The government's desire to deliver (under construction) by the next election so they can continue to grandstand about their infrastructure program.
2) The Auditor General and possibly IBAC digging around the Governments PPP and Market led proposals. The Westgate Tunnel Project was a market led proposal that did not 'require' any real scrutiny. The auditor general reported last year that the same benefits the WGT is delivering could have been done by delivering off ramps on the Westgate that Labor Government took to the election. Cost of the project was a mere $500 Million. A love job was required for Lendlease after the EWL debacle. All of the governments PPP have been slammed by the auditor general as lacking value for money for the tax payer.
3) For once it seem like the state government is looking at the bigger picture, if they were to go down the track (Pardon the pun) of PPP, the 'consortium' would hold control of one of the most critical transport infrastructure items in the states history. The government both state and federal want HSR to the regions, Metro service to Melton and WV and the Airport Link. Handing control of the most critical piece to a private consortium that could gouge the government and inhibit all of these plans is not something the state seems keen to do.
4) Utilising exisiting infrastructure, something that we are not very good at doing here. MM1 has capacity for 32 TPH. Currently 10 TPH pass through Footscray between 6 - 7, The busiest hour. Even if this was to increase you could still have 6 TPH to the Airport and 6TPH to Melton or WV. This, as much as we hate to admit it would suffice for the next decade. The Sunshine - City Tunnel could be built at a later date. Possibly even using the current TBM's from Metro 1 which would be available from 2022. We may not have our tunnel and HSR to the regions until the end of the next decade. However, the government of the day would be free to use the capacity of the new tunnel as it see's fit.
5) The state government has called the feds bluff, unless they are prepared to double their contribution a tunnel is never happening without the help of the private sector. The feds have learnt that the state is very good at getting their way. The feds also have an upcoming election, it will not look good if they are withholding money in the current economic climate. Andrews has with almost certainty won the next election at the last election unless through scandal the government implodes. The Feds are not so lucky and will be looking at trying to get the seats they lost in the last election back. With their current performance in NSW I can't imagine them being the blue ribbon state.
Now on a personal note, why can the state not borrow the entire sum (tunnel and all) and collect the airport fare (minus what the Airport clams as landslide access) and build it alone. With interest rates for borrowing so low and the economy in need of major spending why is there even a need to involve the private sector?