The former Victorian premier who made his political mark as a debt-busting crusader for small government and privatisation has urged the Napthine government to borrow whatever it takes to build a comprehensive underground rail system for Melbourne.

Former Liberal leader Jeff Kennett told Fairfax Media that an expanded metro network was Melbourne's top infrastructure priority. ''There's not a major world city that doesn't have one.''

He said future generations would appreciate that earlier governments had had the foresight to borrow while interest rates were low and build public transport. ''In 100 years people won't care about how much it [the rail system] cost.''

In comments likely to rile the Labor adversaries he flayed for fiscal profligacy, Mr Kennett said debt was good if used for infrastructure, and money was cheap. ''The public sees debt as a no-no. The public needs to be educated about debt.''