The Morrison government may be facing a tougher battle to produce the nation's first federal budget surplus in more than a decade, with new figures pointing to an unexpected drop in GST and company tax revenues.

Finance Department figures released on Friday showed that at the end of December, the budget was $1.1 billion behind expectations and still deep in deficit.

New figures show the federal budget $1.1 billion behind its expected trajectory to surplus before the full impact of the bushfires and coronavirus has come through. Credit:AAP

In the mid-year budget update, released in mid-December, the government was forced to downgrade its expected surplus for the 2019-20 financial year to $5 billion from an original forecast of $7.1 billion. Much of that was driven by a fall in expected tax revenues, with $3 billion written off for the current financial year and $32.6 billion over the forward estimates.

Despite continuing strong iron ore prices, the Finance figures revealed company tax revenues at the end of December were $516 million lower than had been forecast in the mid-year update, while personal tax collections were $379 million behind expectations.