South Australian Greens Senator Sarah Hanson-Young admits she faces a tough battle for re-election after Independent Senator Nick Xenophon confirmed he would not direct preferences her way.

Senator Xenophon says he has been swamped with calls in the past week from supporters urging him not to give first preference to the Greens above the line on ballot papers.

Senator Hanson-Young relied heavily on preferences when she was elected in 2007.

She says she will have to work hard to boost her primary vote.

Senator Hanson-Young says after six months of negotiations she had hoped to reach a deal with Senator Xenophon and is disappointed he will now preference the major parties.

"Nick made it very clear a week or so ago to us, particularly to Bob Brown, that's what he was going to do," she said.

"If he wants to preference the big parties over the Greens and other minor parties that's his choice.

"We've been worried about this for some time. I've been saying up front and clearly that Tony Abbott is coming to South Australia to get my seat.

"He wants it because he wants to get total control of the Parliament. He can only do that if he knocks me off and Nick has given him a help in that regard, he's given him an absolute leg up."

'Strong message'

Senator Xenophon says many of his supporters vote for the major parties in the lower house and do not want him preferencing the Greens.

He says he was surprised by the backlash after former Greens leader Bob Brown called on him for his preferences.

He says Mr Brown's comments have left him in a difficult position.

"[Bob Brown] put out a statement saying 'Nick has to preference us and he shouldn't be doing anything with anyone else' or words to that effect," he said.

"As a result of that my campaign officers got hit with literally hundreds of phone calls, emails, overwhelmingly saying 'we don't want you to do that'.

"They said 'we're happy to vote Coalition in the lower house or vote Labor in the lower house but in the Senate we don't want you to put your preferences above the major parties' and that's been a very strong message. I haven't done any deals with the major parties but I've had to make the stand that I did because my new volunteers to the office were actually drying up after those statements were made."

Senate candidates have been formally finalised today.

A spokesman for the Electoral Commission says candidates will have 24 hours to decide on preferencing arrangements.