A language barrier between two German nationals and a "very good" police officer working their second shift as part of the airport operation led to the returned travellers being allowed to fly on to Melbourne after landing in Sydney on Saturday.
The German woman and her son have both tested negative.
NSW Police Deputy Commissioner Jeff Loy said the officer in question was new to the operation at Sydney Airport.
"It was his second shift in the operation and there was some confusion," he said.
The police officer, an active supervisor, mistakenly thought the pair had an exemption to continue travelling to Melbourne.
"The officer contacted the police operations centre and confirmed what the process would be under that exemption and those people were then taken to the domestic airport in Sydney and flew to Victoria.
"Once in Victoria, it was identified at the airport that these two people did not have the appropriate exemptions and of course the system then went into place and they were taken into the quarantine operation down in Victoria," Deputy Commissioner Loy said.
He clarified that the travellers did not lie to police and it was just a misunderstanding.
He said the officer in question was "very remorseful about the mistake he's made. But that mistake has been rectified, reviewed and we'll move on."
The travellers and the officer won't face any further punishment.
"It's a police mistake and we own that mistake and we move forward," Deputy Commissioner Loy said, adding that processes at the airport have been "reviewed and strengthened" to avoid a repeat.
