A 31-year-old former nurse has been arrested for murder two years after a series of "angel of death" killings occurred at a hospital in Yokohama.

Ayumi Kuboki has reportedly told police that she injected a chemical used in disinfectants into the intravenous drips of at least 20 of her patients. Another 10 unused IV bags had also been found with puncture marks.

The deaths occurred between July and September 2016, when Kuboki was working at the 85-bed Oguchi Hospital for the elderly, now renamed the Yokohama Hajime Hospital.

"I hated seeing the rapid deterioration in their condition," Kuboki was quoted by TBS News as telling investigators.

"I did not want them to die during my shift. It was troublesome and difficult having to explain to their family members."

She was nabbed last Saturday after traces of the chemical were found on her nurse's uniform, and has confessed to the crimes during voluntary questioning by the police.

Forty-eight people died at the hospital during the three-month period, including five patients in one day in August 2016, and another four in a single day a month later.

Police have confirmed that at least four of them died under suspicious circumstances, though it is said that it would be difficult to ascertain the precise cause of death of many of the victims as their remains had been cremated.

Hospital officials had not suspected that anything was amiss at first, owing to the old age of the patients and their debilitating health condition when they were admitted.

But the alarm was raised when a nurse noticed the abnormal presence of small bubbles in the IV drip bag used by Mr Sozo Nishikawa, 88, on Sept 18, 2016.

Two days later, Mr Nobuo Yamaki, 88, also died of poisoning in the same ward.

Ironically, their deaths bookended the Respect for the Aged Day, a public holiday.

The families of Mr Nishikawa and Mr Yamaki told public broadcaster NHK that they are relieved an arrest has been made.

Mr Yamaki's eldest son was quoted as saying: "I want to know why a person who found herself fit to work as a nurse could commit such a crime."