Mujinga, who had an underlying health condition, worked for Govia Thameslink Railway (GTR) on the station contour at the time of the incident on March 22.
The British Transport Police said in a statement to CNN that an investigation was underway into the incident.
"Belly and her colleague asked to be allowed to work from the building with a protective barrier between them and the public for the rest of the day," TSSA said in the statement.
"The management said they needed people who worked outside and sent them out again to the conference for the rest of the shift."
Both women went outside and completed the shift, the union added, but they did not have any personal protective equipment.
Mujinga had underlying respiratory problems for which she had had surgery, regular hospital appointments and had previously had to take time off from work, according to TSSA.
The union says GTR knew about her condition, and even after the incident, Mujinga only stood down after her doctor called her work around March 25.
Manuel Cortes, TSSA Secretary General, accused GTR of not taking Mujinga's abuse "seriously enough" and criticized the company.
"As a vulnerable person in the & # 39; danger zone & # 39; category and her condition known to her employer, there are questions as to why GTR did not put her out of frontline work early in this pandemic," Cortes said in the TSSA statement. "There are serious questions about her death, it was not inevitable."
GTR is investigating the allegations, it said in a statement to CNN on Tuesday, adding that they take the allegations "extremely seriously."
"The safety of our customers and staff, who are themselves key workers, continues to be cautious at all times, and we follow the latest governmental advice," Angie Doll, CEO of Southern Railway and Gatwick Express, said in the statement.
"We are devastated that our dedicated colleague Belly has passed away and our deepest sympathy is with her family, whom we have been in contact with during this very difficult time," she also said.
Both Mujinga and her colleague fell ill from Covid-19 within days of the assault, according to TSSA.
On April 2 – just 11 days after the incident – Mujinga was taken to Barnet hospital, north London, in an ambulance and put on a ventilator.
"Her stomach died April 5, 14 days after she was attacked at Victoria Station," TSSA said.
She leaves behind an 11-year-old daughter and a husband, who last saw her when she was taken away by ambulance, the union added.
"We are shocked and devastated by Belly's death," Cortes said in the TSSA statement. The union said that they take legal advice on the situation and support Mujinga's family and colleagues.
It also reported on the incident to the Railway Authority, the safety arm of the Office for Road and Rail (ORR).
An ORR spokesman told CNN that the office is investigating the incident.
TSSA revealed the history of Mujinga when the British government urged some people to return to work if they were unable to do so from home, and eased some of the restrictions it had put in place to stop the spread of Covid-19.
The union says there is not enough guidance or protection for frontline workers, such as Mujinga.
"Rather than talk about facilitating the lockdown, the government must first make sure the proper precautions and protections are taken so that more lives are not lost," Cortes said.
"Our rail industry needs to take a very serious look at what tasks are considered" essential "and must put in place protection for all our members and our passengers."
TSSA also asked the government to take further measures to compensate frontline workers from the railway industry for their work during these difficult times.