At least $1 billion in funding still needs to be found to complete the Parramatta light rail project, leaked internal documents show, meaning the Berejiklian government could be forced to double the money it has reserved in infrastructure funds.

A cabinet-in-confidence document dated March 2017 obtained by the opposition reveals the total cost of the project at $3.41 billion – $2.41 billion more than the $1 billion set aside from the Restart NSW and Rebuilding NSW funds.

Separate documents from May last year, obtained by Fairfax Media, reveal Transport for NSW estimates $1.3 billion will be raised from "value capture" – a levy on property development along the rail line previously put at $200 per square metre of gross floor area.

The Transport for NSW Finance and Investment committee document states: "Balance of funding to be sought from other funding sources including Federal."