Budget leak reveals Labor short $20bn in funding for spending promises

9NEWS political editor Laurie Oakes has revealed Labor will have to find an extra $20 billion in funding after a leaked federal budget document showed a shortfall in the opposition's expected revenue from taxing smokers. 

In the document obtained by 9NEWS, the Treasury forecasts the tobacco excise will raise $28.2 billion over 10 years – $19.8 billion less than Labor had predicted over the decade. 

It’s believed Labor will consequently be left short on funding for its spending promises, which include fully funding the Gonski school model. 

The document will be attached to tomorrow night’s budget papers and will be tabled by the Treasurer Scott Morrison.