The State Government prepared no thorough business case before committing to a revamped route for Melbourne's city rail tunnel, instead relying on "common sense".

Planning Minister Matthew Guy made that startling admission on Thursday under questioning about why the government had dumped plans for a rail tunnel linking Parkville with South Yarra in favour of one that creates a new train station at Montague, near South Melbourne.

The blue line shows the existing rail lines; the orange line the Melbourne Rail Link -Tunnel; and the green the Melbourne Rail Link-Airport Rail Link that continues to Melbourne Airport.

The government says the new station will service the planned new mega suburb at Fisherman's Bend, despite being kilometres away from that location.

The opposition has claimed the plan is the result of last-minute panic from a government trying to save its electoral skin.