Romania's centrist President Klaus Iohannis easily won a presidential runoff vote as expected, crushing his socialist challenger on a pledge to restart a judicial reform slowed down by successive Social Democrat (PSD) governments.

Iohannis had been the short-odds favourite to beat former PSD prime minister Viorica Dancila comfortably in Sunday's runoff.

An exit poll conducted by IRES pollster showed Iohannis garnered 66.5 percent of votes followed by Dancila with 33.5 percent. Another pollster CURS-Avangarde showed Iohannis winning with 64 percent.

Under a succession of PSD governments, Romania rolled back anti-corruption measures and weakened the independence of the courts. Along with ex-communist peers Poland and Hungary, it has been heavily criticised by Brussels for its actions.

However, the 60-year-old Iohannis has been credited by Western allies and the European Union with trying to protect the rule of law, in particular by challenging attempts to limit judges' independence.

The president's powers are mostly limited to nominating a prime minister based on who can command a majority, challenging laws in the Constitutional Court and appointing some chief prosecutors.

Iohannis will have a chance to install anti-graft and anti-mafia prosecutors who are serious about tackling endemic corruption with the backing of Prime Minister Ludovic Orban, who became head of a liberal minority government by winning a parliamentary vote of confidence three weeks ago.

Teacher Andreea Mihai, 50, said earlier if Iohannis won on Sunday, "things should slowly return to normality.

"Both Orban and Iohannis will work together in the same direction," Mihai said.

Dancila's PSD had increased the burden of proof in corruption cases, reorganised panels of judges and set up a special unit to investigate magistrates for potential abuses, a move widely seen as an instrument of political coercion.

Romania's judicial reforms have been monitored by Brussels since it joined the EU in 2007; in October, Brussels said the reforms were going backwards.

Iohannis, a soft-spoken ethnic German and former mayor of Sibiu, became president in 2014.

He helped to secure a popular approval in a referendum last May that called for the government to be banned from altering legislation by emergency decree, and advocated a ban on amnesties and pardons for graft-related crimes.