Japan's public broadcaster has vowed to reform its working practices as it revealed that a young reporter died of heart failure after logging 159 hours of overtime in a month.

NHK reporter Miwa Sado, 31, who had been covering political news in Tokyo, was found dead in her bed in July 2013, reportedly clutching her mobile phone.

A year later, Japanese authorities said her death was linked to excessive overtime. She had two days off in the month before she died.

NHK eventually made the case public four years afterwards, bowing to pressure from Sado's parents to take action to prevent a recurrence.

The case again highlights the Japanese problem of karoshi, or death from overwork, amid the country's notoriously long work hours.

It is also an embarrassing revelation for NHK, which has campaigned against the nation's long working culture.

Sato covered Tokyo assembly elections in June 2013 and an upper-house vote for the national parliament the following month.

She died three days after the upper-house election.

"My heart breaks at the thought that she may have wanted to call me" in her last moments, her mother told the Asahi daily.

"With Miwa gone, I feel like half of my body has been torn off. I won't be able to laugh for real for the rest of my life."

The revelation shocked the nation as NHK has actively reported tragic deaths at other companies, including the 2015 suicide of a young woman at major advertising agency Dentsu after logging more than 100 hours of overtime a month.

The chief of NHK has pledged to improve work conditions at the broadcaster.

"We are sorry that we lost an excellent reporter and take seriously the fact that her death was recognised as work-related," President Ryoichi Ueda said Thursday.

"We will continue to work for reform in cooperation with her parents," he told reporters.