Djokovic arrived at Melbourne's Tullamarine airport on Wednesday night but was detained by the ABF after it was determined he did not meet the entry requirements for an unvaccinated traveller.

Hours earlier, the ABC had revealed the acting ABF Commissioner Dwayne Freeman had begun examining an "issue" with the Australia Travel Declaration submitted by the tennis star before he flew into Melbourne.

Late on Thursday, an ABF spokesperson confirmed that documentation provided by another two individuals who recently travelled to Australia was also being examined to determine if they meet entry requirements for unvaccinated arrivals.

"The ABF is aware of two other individuals who have used the same exemption letter issued by Tennis Australia as the basis for their medical exemption for the purposes of travelling to Australia," the spokesperson told the ABC.

"ABF is making inquiries regarding these individuals, as the investigation is ongoing, we won't be making any further comment at this time."

An official with knowledge of the Djokovic case said authorities began closely examining the Serbian's circumstances after he declared on social media on Tuesday that he was "heading Down Under with an exemption permission".

The 34-year-old had tried to enter Australia on the basis that he had contracted COVID-19 in the past six months and had been provided a valid exemption by Tennis Australia for being unvaccinated.

Government sources say Djokovic handed Border Force officials a medical exemption on Tennis Australia letterhead that was signed by the organisation's Chief Medical Officer, but this was rejected.

In November, Health Minister Greg Hunt wrote to Tennis Australia CEO Craig Tiley warning that players who sought to enter the country would not be granted vaccine exemptions if they had recently contracted COVID-19.

"The Australian Border Force has advised that people must be fully vaccinated, as defined by the ATAGI, to gain quarantine-free entry into Australia," Mr Hunt wrote on November 29 last year.

"I can confirm that people who contracted COVID-19 within the past six months and seek to enter Australia from overseas, and have not received two doses of a Therapeutic Goods Administration (TGA)-approved or TGA-recognised vaccine (or one dose of the Johnson and Johnson COVID-19 vaccine), are not considered fully vaccinated".

Speaking to Radio 2GB, Home Affairs Minister Karen Andrews confirmed Border Force was now investigating two more recent arrivals who had Tennis Australia paperwork and didn't rule out they could be sent back home.

"I'm aware of those allegations, and I can assure you that the Australian Border Force is investigating that now," Ms Andrews said.

"ABF needs the opportunity to be able to conduct its investigation, but if the evidence is not there, then they will take the appropriate action."

Story By | Andrew Greene