Brazil's electoral tribunal on Thursday fined the ruling Workers Party (PT) of President Dilma Rousseff $1.6 million for accounting irregularities dating back to 2009.

It comes with the party already reeling from the fallout of a giant graft scandal at state-owned oil firm Petrobras which has seen some three dozen politicians, including the PT's former treasurer, fingered by a police investigation into alleged kickbacks on fat Petrobras contracts.

Thursday's ruling locks the PT out of access to state party funding for three months and the organization will have to pay back the cash concerned, the tribunal said in a statement.

The investigation found irregularities surrounding the payment of outstanding interest as well as incomplete documentation on tax and income obtained from unidentified sources.

Rousseff was re-elected last October despite the growing controversy over the Petrobras scandal, which has rocked the political establishment and seen her personal poll standings slump.

Joao Vaccari Neto, the PT treasurer, was forced to stand down following his arrest on charges linking him to the kickbacks affair, which Petrobras estimates cost the company $2 billion.