Credit ratings agency Fitch surprised analysts by upgrading Malaysia's sovereign outlook to "stable" from "negative", boosting the country's currency on Wednesday and providing welcome relief to the country's controversy-plagued government.

In a statement just before midnight on Tuesday, Fitch affirmed the country's credit rating at "A-", the fourth-lowest investment rating, and said Malaysia's fiscal finances had improved since 2014, citing progress on a newly-introduced consumption tax and fuel subsidy reforms.

"Malaysia's rating remains supported by reasonably strong real GDP growth rates and low inflation volatility," said Fitch, which had downgraded Malaysia's sovereign outlook to "negative" in 2013.

The positive news helped bolster the local currency on Wednesday, with the ringgit at 3.7347 against the US dollar, up from 3.7733 on Tuesday.

"It (the outlook revision) is a nice surprise," Nicholas Teo, an analyst at CMC Markets in Singapore, told AFP.

"We are seeing a big bounce on the ringgit now, presumably on the back of this."

IG Markets strategist Bernard Aw characterised the news as a "180-degree turn", noting that "the market had been bearish towards the outlook with sentiment being driven by fears the rating would be downgraded".

"The underlying fundamentals of Malaysia's economy are still stable," he added.

Rising government debt, mounting stress on the ringgit and weakening oil prices have been denting Malaysia's economy this year.

The Fitch statement will also be welcomed by Prime Minister Najib Razak, who is facing pressure to get Malaysia's financial house in order.

Along with having to deal with public anger over a newly-introduced consumption tax, Najib, who is also the country's finance minister, has been struggling to fend off a persistent campaign for his ouster by influential former premier Mahathir Mohamad.

Najib was weakened by 2013 polls in which Malaysia's long-ruling coalition nearly lost power, and his reputation has been hammered this year by a drip-feed of damaging allegations of fraud at a debt-ridden state investment company he oversees.

The prime minister has skirted demands that he explain how hundreds of millions of dollars have allegedly gone missing from deals involving 1Malaysia Development Berhad (1MDB).

He has denied wrongdoing and ordered government auditors to examine 1MDB's books.

Malaysia's finance ministry on Wednesday called Fitch's statement "fair and balanced".