The delivery office on Pitt Street closed on Wednesday, alongside the Grimethorpe and Cudworth offices, after positive tests for the virus were confirmed throughout the week.
This followed the deep clean that the delivery office underwent on Monday, which saw workers stood outside the building, on Pitt Street, from 8am as the emergency clean was carried out - which subsequently ended in a meeting before they were allowed back in.
It follows reports of deliveries from the building being seriously affected over the past few weeks due to the ongoing pandemic.
A Royal Mail spokesman said: “Royal Mail takes the health and safety of its colleagues, its customers and the local communities in which we operate very seriously.
“Following further positive tests for coronavirus at the Barnsley delivery office, we have taken the decision to close the office until Friday as a precautionary measure.
“During this time we will carry out an additional intense, enhanced clean of the building.
“We are working to keep disruption to mail deliveries to a minimum as we address the concerns of our colleagues.
“As part of this process, we are diverting mail to another nearby unit.”
Several precautions have been put in place by Royal Mail to ensure the safety of its employees, with the company keeping in touch with the Communication Workers’ Union (CWU).
“Throughout this crisis, every decision we make puts the health of our people and customers first,” the Royal Mail spokesman added.
“We have put in place a range of preventive measures to protect both our customers and our colleagues.
“As well as encouraging good hand hygiene, standard ways of working have been revised to ensure that colleagues stay two metres apart at all times.
“All staff have been briefed about the social distancing measures jointly agreed by local management and the CWU.
“This has been supplemented with visible reminders such as posters and one-way floor markings.
“In addition there is only one person permitted in a Royal Mail delivery vehicle at any one time.”
The CWU were involved with ensuring the deep clean was satisfactory, and workers were allowed back into the building during the afternoon.
Treasurer Mick Dunlavey added: “Over the weekend at the Barnsley delivery office there were a number of confirmed Covid-19 positive tests.
“As health and safety is paramount for our members, it was agreed that a deep clean would take place on Monday morning, and as a precautionary measure no-one was allowed in.
“The clean was completed by 1.30pm and we are organising a delivery plan for workers to get back to work.”
Due to the staff being unable to get back into the building, there was ‘serious disruption’ to the postal service across the region.
A Royal Mail spokesman added: “We’re sorry to advise that due to resourcing issues, deliveries of mail in the S70, S71, S71 and S75 postcode areas will be limited again today, June 24.
“As a result, our focus today will be on delivering as many special delivery and tracked items as possible.
“We apologise for any inconvenience this may cause you.”