Taxpayers could be forced to pay extra fees to Transurban if there are delays in building the $6.7 billion West Gate Tunnel toll road, official documents show.

But it's not known how much compensation the state has promised the company or what constitutes a delay.

Transurban has already told its investors that it would be paid extra by taxpayers if the government breaks a key promise in striking the West Gate Tunnel deal - that the CityLink tolling contract would be extended by an extra 10 years.

The Andrews government has not disclosed this compensation fee, but finance giant JP Morgan estimated that it would be at least $1.8 billion.