The bank had cut its key interest rate in October by 50 basis points, aiming to stimulate investment and boost faltering growth. It also cut the rate by 100 basis points in March in response to the coronavirus outbreak.
The central bank will finance the supplementary budget for 2020 by $1.1 billion by purchasing treasury bonds after an exceptional license from Parliament last week.
But the bank said in statement that "it will be more vigilant to intervene, if necessary, in order to contain the potential effects of this measure, in particular on inflation and local currency."
It urged speeding up the implementation of reforms aimed at curbing abuses and preventing further slippage in public finances in the future.
Tunisia's public finances are dire, with the government now forecasting a record budget deficit of 11.4% of GDP in 2020.
($1 = 2.7218 Tunisian dinars)
(Reporting by Tarek Amara in Tunis; Editing by Kevin Liffey and Matthew Lewis)