A busload of tourists from a western Sydney hotspot visited Parliament House in breach of ACT's travel restrictions, police believe.
Authorities believe the tour group entered and left the ACT on Tuesday and did not visit any other locations.
Anyone caught flouting ACT's COVID-19 restrictions can face fines of up to $8000.
An ACT Policing spokeswoman said the force was working with ACT Health to investigate the matter.
"This visit was not related to the peaceful protest that occurred at the forecourt earlier in the day," she said.
"ACT Policing would like to remind the community to follow all public health directions and COVID-19 protocols, including quarantine and self-isolation directions."
Health Minister Rachel Stephen-Smith said the government took breaches of public health directions seriously.
"Public health directions are there for a very good reason" she said.
"When people are asked not to travel to a particular area or they are asked to quarantine, or required to do so by a public health direction, we expect people to abide by that."
But Ms Stephen-Smith said the Chief Health Officer believed the risk to the public from the potential breach was low.
"That really reflects the improving situation in greater Sydney at this point in time," she said.
"The Chief Health Officer did indicate at our briefing last Friday that she would be considering this week removing the Cumberland local government area as the final hotspot in terms of travel restrictions to the ACT."
NSW has not recorded any locally acquired COVID-19 cases for 10 days.
The group was reportedly asked for their postcode at the conclusion of the tour of Parliament before security raised the alarm.
A Department of Parliamentary Services spokesman confirmed deep-cleaning of affected areas was completed on Tuesday.
"DPS is aware a group of people potentially from a designated COVID-19 affected area of NSW visited the ACT yesterday, against ACT public health directions, and including a visit to Parliament House," the spokesman said.
"DPS has been in contact with ACT Health and provided relevant information to authorities as well as undertaking cleaning in line with COVID-safe plans for the building."
The Cumberland LGA is the only area currently with restricted travel to the ACT.
Parliament's COVID-19 protocols were last revisited in November, before the NSW outbreak. Presiding officers Speaker Tony Smith and Senator Scott Ryan directed that mask wearing was not mandatory but was strongly recommended in areas of the building where people circulate.
Seven News initially reported the group may have also visited the Australian War Memorial and the group could have spent up to three days in the capital.
A spokesperson from the Australian War Memorial said ACT Health advised the Memorial to continue operating under its current COVID-Safe plan without the need for additional actions.
"The Memorial continues to remain proactive in our policies, procedures and guidance material to visitors and will follow any updates from ACT Health on this emerging development," the spokesperson said.
"Public surface areas are regularly sanitised during visitor hours with a thorough clean of the Memorial at the conclusion of each day."