A worker from Costco's head office in Sydney has tested positive for COVID-19 after working four days at the company's new Boolaroo warehouse.
Hunter New England Health has identified more than 300 close contacts of the person, putting the region's emergence from lockdown next week under threat.
A Costco representative said on Friday that the employee had infected two co-workers.
Deputy Premier John Barilaro said the person had been unwell at work for two days and had "done the wrong thing".
Costco confirmed the employee had been "highly infectious" while at work on Wednesday and Thursday and had gone to work on Thursday when they should have been isolating while awaiting a Wednesday test result.
The Boolaroo store was due to open on August 28, but Costco said it would likely delay the opening for four weeks.
Hunter New England Health had earlier identified the original case as a construction worker, but the warehouse's building contractor told the Newcastle Herald that it had handed over the building to Costco two weeks ago.
The Costco employee was on site from Monday to Thursday this week training staff for the opening.
The company representative said the person had received a negative test before travelling to Newcastle from Parramatta on Monday.
The employee had been tested again on Wednesday.
HNEH public health controller Dr David Durrheim said he expected more positive cases from the Costco site.
Anyone who attended the Costco on-site membership office and petrol station at Boolaroo any time from Monday to Thursday is considered a close contact and must get tested and isolate immediately.
The order will affect any Costco members who picked up their membership cards at the site this week.
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