The diplomat in his 50s flew in to Sydney on April 25, travelling to Canberra by private vehicle on the same day.

The man reportedly travelled directly to Canberra and has been in home quarantine since his return.

ACT Health reported the man made no stops en route from Sydney and he had followed all requirements since his arrival.

"While in quarantine, the diplomat experienced mild COVID-19 symptoms and was tested for COVID-19, returning a positive result," a spokesperson said.

"The individual has followed all quarantine measures for returning overseas travellers and is being supported by ACT Health."

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ACT Health said it had identified three close contacts of the diplomat through contact tracing.

The close contacts were also in quarantine and there was no risk to the public, ACT Health said.

The NSW government had been informed of the positive case to allow for contact tracing.

The positive case would be counted in ACT Health statistics reporting on Thursday and genome sequencing would be undertaken to determine whether the man was infected with a variant of concern.

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"The ACT receives returning diplomats and government officials on a regular basis and given the COVID-19 situation internationally, it is not uncommon for there to be some reports of positive COVID-19 cases in the ACT's quarantine system," an ACT Health spokesperson said.

"This is exactly why the ACT has a quarantine system.

"The circumstances of this new case demonstrates that our quarantine system here in the ACT is working well to protect the Canberra community from COVID-19."