"We've got more than 3000 close contacts identified from those various exposure sites, and known contacts already," Ms Stephen-Smith told 2CC radio.

"Wednesday evening it was positive wastewater; yesterday morning it was one case; by yesterday afternoon it was four cases: This is Delta strain," she said of the ACT's snap decision to lockdown for seven days from Thursday at 5pm.

Case investigations to identify the source of the first infection were still ongoing yesterday afternoon, Ms Stephen-Smith said.

"The genomic sequencing will come back today, we're expecting it around lunchtime, and that will really help to identify whether there is a link to an existing known case," she said.

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"We expect that will probably be in the Sydney outbreak but that will be really helpful piece of additional information."

The snap lockdown was announced Thursday after the ACT recorded its first COVID-19 case infectious in the community in more than a year.

Three new cases, all close contacts of the first known case, were announced Thursday evening, bringing the ACT total to four.

ACT Health is updating a list of exposure locations, and experts told The Canberra Times that a nightclub and a church on the list are of particular concern.

"We know that this virus can spread in really unusual manners, regardless of whether it's Delta. It depends on the airflow," UNSW epidemiologist Professor Mary-Louise McLaws said.

"You don't need to be close to a source, a person infected, when you're indoors like that, with potentially air conditioning airflow."

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