The state government has yet to spend one cent of the $50 million it included in last year's budget to plan and develop its landmark Melbourne Metro rail project.

While Premier Denis Napthine defended the decision to put off the rail project by saying the business case for it was not yet ready, Tuesday's budget papers suggest this is because his own government has spent no money on it.

Rather, the $40 million spent so far to prepare plans for the nine-kilometre underground line has been funded by the Gillard government, which has paid the state to develop the idea to the point at which decisions can be made.

In last year's budget, the state added $49.7 million of its own money to speed up the planning work, of which $11.4 million was to be spent in 2012-13. The money was saved instead, and just $10.2 million has been allocated for the same job in 2013-14.