The entire floor where the person was working has been closed further notice.
Staff working on the floor were sent home to self-isolate and get tested.
This morning, a spokesman updated, "We've had all the close contacts report back as testing negative so fingers crossed that's it."
There have been no reports of Covid-19 infections among Datacom's local staff, the spokesman said.
Datacom has around 6500 staff across New Zealand, Australia, Malaysia and the Philippines.
The Sydney staffer was the company's first and only positive case.
Datacom was one of the first companies to send most staff home as the pandemic started to take hold - on the logic it had to get its own house in order before assisting clients who were shifting to remote working.
"We sent around 90 per cent of staff home when Covid first hit," the spokesman said.
"Those that remained had to for security reasons or for datacentre support. We had one guy in NZ move into his campervan on-site at a data centre," the spokesman said.
Australia had 21 new cases of Covid-19 yesterday, with five in New South Wales (two quarantined at the border two in the community and one under investigation). There are 26 active cases in the state.
Full pay for isolated staff
Datacom chief executive Greg Davidson said, "While we have a larger percentage of our staff working remotely, as an essential service for our customers requires some of our staff to work in an office."
Datacom has put in place strict protocols and for essential workers who are required to be in an office, enhanced cleaning regimes including deep cleaning and fogging, and has modified the office to ensure we minimise risk to our staff, the CEO said.
"We are monitoring the issue closely, working with NSW Health to ensure any further actions are taken. We have activated our pandemic response plan and senior business leaders are actively managing the situation from our end.
"The wellbeing of our staff is our priority. Anyone who is asked to self-isolate as a precautionary measure, will do so on full pay.
"We have also made our employee assistance services available to those who are finding these events difficult to handle."
Cautious outlook with Covid
Datacom - 60 per cent owned by the rich-list Holdsworth family, and 40 per cent by the NZ Super Fund - recently reported a 3.1 per cent increase in revenue to $1.34 billion for its 2020 financial year, although its net profit halved to $19m, in part because of the impact of Covid-19 on the fourth quarter.
"One of the things that no one can predict is quite how deep the recession will be", Davidson said on July 30 as the financials were posted.
"What we do see is that everybody's horizons have shortened.
"A lot of people spent a lot of their budget for the year on helping make remote working happen for all of their teams.
"And they're now only wanting to commit to shorter-term, more obvious payback-type activity for the foreseeable future. So really, it's about us being on our toes."