The situation has infuriated customers awaiting online orders and US businesses that regularly send mail to Australia.
Former Porsche dealer Alan Hamilton is selling part of the historic Bryant&May match factory.
Workers at the company went on strike for 24 hours on Thursday as they bargain for more job security.
The 24-hour strike comes a month after thousands of employees at transport giant Toll walked off the job and is set to impact already-strained supply chains.
Parcel volumes are already at Christmas-like levels due to more than 15 million Australians living under lockdown.
The TWU said 90 per cent of StarTrack union members voted in favour of a strike but the company said less than one-third of its workforce supported the action.
Retailers have been on the receiving end of customer discontent over Australia Post’s delivery delays as the country’s parcel services slowly start to improve.
Australia Post’s interim chief executive,Rodney Boys,has flagged some products could be in short supply this Christmas,as COVID takes a toll on global supply chains.
The $1.8 million underpayment draws further attention to the postal service’s pay practices,which have been in the public eye in recent times.
A total of 345 employees received an average bonus of almost $82,000 last year,with Australia Post splashing almost $92 million on its staff.
More than a third of customers have not been complying with coronavirus safety rules in recent weeks,putting staff safety at risk,the union says.