The announcement followed the cancellation a day earlier of a regional summit on good governance to be hosted by Fiji. Australia had lobbied for the meeting to be called off.
"We will be making very, very clear to Fiji our protest about this unreasonable and uncalled for action"
Julia Gillard, Australian prime minister
In an interview with New Zealand radio, Frank Bainimarama, Fiji's military commander, blamed Australia for persuading Vanuatu to cancel the meeting of the Melanesian Spearhead Group (MSG), which includes Papua New Guinea and the Solomon Islands.
Bainimarama charged Australia and fellow South Pacific neighbour New Zealand with plotting to embarrass his country.
He said he was "seriously thinking" about cancelling a return to democratic elections in 2014 because of "interference by these people".
Stephen Smith, the Australian foreign minister, described the claim as "just a ruse", adding he would not be surprised if there was no election in 2014.
"But we want elections before then," said Smith.
Edward Natapei, Vanuatu's prime minister, said the summit which is currently chaired by his nation, had been postponed because of the potential long-term ramifications of allowing Fiji to host it.
"There are basic fundamental principles and values of democracy and good governance that our organisation is built on and we must continue to uphold them," he said in a statement.
Roberts is the second senior Australian diplomat to be expelled from Fiji in less than a year, after the previous high commissioner and his New Zealand counterpart were removed last November over alleged interference in Fiji's judiciary.
Australia's prime minister also condemned the expulsion.
"We will be making very, very clear to Fiji our protest about this unreasonable and uncalled for action," Julia Gillard said in Canberra.
Australia and New Zealand have been Bainimarama's loudest international critics since he led a bloodless coup four years ago, and have pressured him to restore democracy.
Since the 2006 coup, Fiji has been suspended from the Commonwealth and the 16-nation Pacific Islands Forum and has been hit with sanctions by the European Union and countries including the United States, Australia and New Zealand.