Mr Downer did not respond to calls on Sunday,but one of his predecessors as high commissioner,former senator Richard Alston,said there was nothing unusual about Mr Downer's meeting with Mr Papadopoulos.
"We all mix with interesting people at times,"he said."You do follow leads,and I don't find anything particularly odd about that. You don't necessarily know that someone's a low-level lackey until you've met them,or even afterwards."
TheTimes reported the meeting came about through a series of connections involving an Israeli diplomat who introduced Mr Papadopoulos to an Australian counterpart. It said the story was sourced from four current and former American and foreign officials.
John Blaxland,professor of international security and intelligence studies at the Australian National University,said Mr Downer would not have taken the revelation lightly and it was"highly likely he would have written a cable or made a phone call".
But it was also likely Australian officials would have wanted to know more - or needed a clear reason - before passing the information on to the Americans,he said.
"Australia is not renowned for meddling with US politics or for dobbing people in in US politics. You tread very,very warily,"Professor Blaxland said."It would be highly unusual for Alexander Downer to act on something like that unless he had pretty good reason."
Mr Alston said intelligence officials would have been responsible for decisions about informing the Americans,not Mr Downer.
"You tell those people and then you leave it up to them. You don't ask whether they've passed it on - they probably wouldn't tell you anyway,"he said."They guard their independence quite jealously. They certainly don't share strategies and things like that with diplomats,even the high commissioner. They do their own thing."
Tom Switzer,a US politics expert,executive director of the Centre for Independent Studies think tank and a friend of Mr Downer,said the revelations amounted to more circumstantial evidence about the Trump camp's links to Moscow and it was"not a good day for Trump".
"Downer's well known in conservative circles in America and I think Papadopoulos probably just thought he'd drop in to say g'day,"Mr Switzer said."He probably fancied himself as a bit of a player when he wasn't actually that big a deal.
"Downer might have smelled a rat and thought this guy's just a bit of a dodgy character. ... Downer's done everything right."
Mr Switzer said he had not discussed the matter with Mr Downer.
In April,the Turnbull government extended Mr Downer's posting in London. He will be replaced by former attorney-general George Brandis,most likely after February,giving time for Senator Brandis to make a valedictory speech.
On Twitter,many Americans told Mr Downer he was a"hero"and had helped to save democracy in the US. In domestic politics,Mr Downer is best known for his disastrous run as leader of the Liberal opposition in 1994,his encounter with a pair of fishnet stockings and his nearly 12 years as foreign minister in the Howard government.