Slow wage growth will help punch an $8.1 billion hole in the Morrison government's budget surplus plan, one of the nation's leading forecasters has predicted amid signs the economy remains subdued in the run-up to Christmas.

Deloitte Access Economics on Monday said the budget is taking a revenue hit while the government prepares to lift spending in key areas including aged care and in response to the drought.

Treasurer Josh Frydenberg handed down the budget a month early in April, predicting a surplus of $7.1 billion which would be the first surplus since 2007-08. That was forecast to grow to $11 billion in 2020-21 and $17.8 billion the following year.

Deloitte Access believes Josh Frydenberg is still on track to deliver the first budget surplus in more than a decade but slow wages growth is making it more difficult. Credit:Alex Ellinghausen

But Deloitte believes slower than expected wages growth is starting to bite into federal revenues.