Dr Padma Lal, who was born in Fiji but is an Australian citizen, told Radio Australia's Pacific Beat program she was given no reason for her expulsion on Tuesday morning.
But she says she believes it is due to her marriage to Professor Brij Lal, a Fijian-born academic who helped write the country's 1997 constitution.
Professor Lal, also an Australian citizen, has spoken criticially of Fiji's interim military government.
He was expelled in November, after making comments to Australian media about Fiji's diplomatic row with Australia and New Zealand.
Dr Padma Lal had been working for the International Union for the Conservation of Nature, based in Suva, on issues of mangrove management, disaster risk management and climate change.
"The work that I'm doing in Fiji or in the Pacific is really for the Pacific and Fiji," she said in a telephone interview from her home in Canberra.
"That's totally apolitical. So as far as I can see, I cannot see any reason why they would detain me other than perhaps (because) I'm married to Professor Brij Lal."
Dr Lal says she was taken aside on Monday at Nadi Airport as she returned from an overseas trip.
"I asked as to why I was not allowed entry and they said well there're no reasons for it," she said.
"Pressing further on that, they basically said we don't need to give any reasons, and then later on they said that you don't have a return ticket. I mentioned that I have a valid visa, and in that valid visa category I don't need a return ticket."
Dr Lal says according to paperwork shown her by immigration officers, her deportation was ordered directly by Commodore Bainimarama
After more than three hours, Dr Lal says she was taken to a hotel room - without a telephone. Her mobile phone and computer were also confiscated.
"There was an immigration officer sitting outside, together with this hotel security person by the looks of it, and the person stayed there all night," she said.
Dr Lal says she was not physically harmed, but was told that she would be detained if she refused to give up her computer.
She says she hopes one day to be allowed to return to Fiji.
As for being deported: "I'm angry and I'm disappointed".
"Angry for obvious reasons, disappointed to see what my beloved country that I was born and brought up in and I still feel quite strongly about", she said.
"Where it's going and the fact that we really do not have freedom of movement, freedom of speech, basic human rights" - Radio Australia