Dozens of residents took to social media to complain about the mystery smell over a week in March, described as being similar to chlorine or sulphur. 

The smell was most pungent at night and could be detected as far away as Carrington and Waratah. 

A spokeswoman for the Environmental Protection Authority (EPA) said it had been making inquiries with various businesses in the Mayfield area, where the odour was suspected to have originated. 

“The odours coincided with a restart of the Koppers facility in Woodstock Street at Mayfield North,” she said. 

“The EPA has requested an Incident Report from Koppers and has undertaken an inspection of the premises.

“The EPA is continuing to investigate these odours and will review the incident report from Koppers before deciding on any further action.” 

The chemical storage facility first came to the attention of authorities in 2012 after a napthalene spill, as a result of a pipe becoming blocked during a cleaning procedure.  

Two years later, the EPA ordered Koppers to complete a number of upgrades to reduce the release of vapours and potentially offensive odours. 

Keith Craig from the Stockton Community Action Group said it was possible the EPA needed to use a “bigger stick” if the upgrades had been unsuccessful in containing the problem. 

Koppers did not respond to the Newcastle Herald’s request for comment.