Nkosi, who was tried and imprisoned for suspected terrorism activities on Robben Island with Deputy President Kgalema Motlanthe in 1977, died after undergoing surgery in 2008 at Charlotte Maxeke Academic Hospital.
The inquest - presided over by Johannesburg magistrate Danie van der Berg - heard that Nkosi underwent elective spinal surgery between 9pm and 9.30pm on July 22 but died shortly after midnight.
According to evidence in court, Nkosi grew restless at around 23.45pm, complaining that he was finding it difficult to breathe.
But, nurses monitoring him only informed doctors of the patient's deteriorating health much later. A doctor only arrived 19 minutes later to resuscitate Nkosi.
Lawyers for one of the doctors, Steven Farrell, said the quality of nursing on that day was "catastrophic" and that a 19-minute window of opportunity to save Nkosi's life was "squandered".
"From a nursing perspective, he (Nkosi) was not properly monitored," Farrell put it to forensic pathologist Dr Hestelle Nel. Nel agreed before adding that she could not accept the nurse's notes as "correct post-operative monitoring" because she could not make out what the nurse - known only as Tshabalala - was recording. Farrell called the nurse's notes "woeful".
Nel again called into question the nurses' monitoring of Nkosi, noting that they should have realised his respiratory rate had jumped to 37 - the normal rate being 12.
She also said increased swelling around Nkosi's neck should have also been a sign that "something should be done". Nkosi suffered from brain swelling because of a lack of oxygen, which caused increased blood flow into the arteries.
The hearing continues.