A Taiwanese steel firm behind a toxic spill that killed tonnes of fish in central Vietnam last year was fined for a second time for illegally burying "harmful" waste, official sources said Sunday.

The deadly dump from Formosa's $11 billion steel plant in Ha Tinh province sparked one of the country's worst environmental catastrophes, decimating livelihoods along swathes of coastline and prompting months of rare protests in the authoritarian country.

The firm was initially fined $500 million for pouring toxic chemicals -- including cyanide -- into the ocean in April 2016, and has now been ordered to pay an additional $25,000 on separate charges of burying harmful solid waste in the ground, according to the official Cong Ly newspaper.

A local contractor will also be fined $20,000 for helping to dispose of the 100 cubic metres of waste, added Cong Ly, the mouthpiece of the Supreme Court.

An official in Ha Tinh province confirmed the latest fine to AFP on Sunday, without providing further details.

The waste was buried in July 2016, and local residents reported seeing trucks ferrying the material to a farm belonging to the contractor hired to dispose of it.

Police confirmed the waste came from Formosa and launched an investigation last year. Officials would not comment on why it took more than a year to issue the nominal fines.

The toxic spill set off angry demonstrations against the company and the government in the one-party state that routinely jails its critics, including by affected fishermen who demanded greater compensation.

Several activists have been arrested and convicted for their involvement in the protests, including a 22-year-old blogger who was jailed for seven years last month.

Formosa's huge steel plant, which was under construction at the time of the disaster, was given the green light to resume operations in April after officials found it had addressed dozens of violations.

Several officials were punished or fired after the disaster, which saw beaches littered with fish, including large offshore species.

Communist Vietnam has been accused of ignoring environmental concerns on its march toward rapid development, though the issue has become a central issue for some groups who have taken up the cause on social media.