Singapore has so far ignored a concerted campaign seeking clemency for Nguyen, who was arrested at Changi airport in late 2002 carrying 396 grams of heroin.
The 25 year old Melbourne man is due to be executed in Singapore on December 2 but his lawyers are hoping the case can be taken to the International Court of Justice.
An application for the case to be heard by the ICJ must be made through the Australian government, but Mr Downer today was pessimistic of the court agreeing to hear Nguyen's case.
"My own preliminary view is that it would be almost impossible to bring a case to the International Court of Justice because you would have to have Singapore's agreement to do so," Mr Downer told reporters in Adelaide.
"The chance of Singapore agreeing to a case being brought to the International Court of Justice are fairly obviously remote given the position they have taken on the execution of Nguyen.”
"My advice is that given Singapore doesn't accept the jurisdiction of the International Court of Justice, except in a very narrow range of areas which are not relevant to Van Nguyen's case…” he said.
"And that is remote in the extreme that they would do that, bearing in mind that they have heard all the arguments in relation to clemency for Van Nguyen."
No false hope
Mr Downer reiterated that he does not want to give Nguyen’s family false hope.
"We always say we’ll never give up you know, it's not the Australian way to give up but it's also not fair on Van Nguyen, his family or anybody else who’s passionate about this case to offer people false hope," Mr Downer said.
"I can't offer people false hope, the chances of us saving him now are very very remote."
Mr Downer has also rejected imposing economic sanctions on Singapore in protest at the scheduled execution.
"Imposing economic sanctions would be a classic case of cutting off your nose to spite your face," he said.
"It would damage Singapore but it would damage Australia, it would damage our commercial interests.
"And in doing so, we would make no contribution whatsoever to helping save Van Nguyen so it would be a lose-lose policy.
Prime Minister John Howard earlier ruled out the possibility of trade sanctions against Singapore.
Van Nguyen’s mother had to be physically supported by friends and their legal team as she and her other son boarded a flight to Singapore this evening.
Nguyen's mother Kim and twin brother Khoa flew out of Melbourne to visit van on death row before he is executed next week.
Ms Nguyen, red-eyed and silent, kept her head down while Khoa walked through the Qantas check-in and into customs, as supporters with them offered words of assurance.
It will be the first time Khoa has seen his brother since he was arrested almost three years ago trying to board a flight to Australia.
Nguyen says he was trafficking heroin to help pay off his brother's legal fees.