There will be an empty seat in the Senate next week, while Bob Carr's replacement Deb O'Neill waits for NSW Parliament to endorse her for the casual vacancy.
State parliament is expected to have a joint sitting on November 14 to sign off on the ALP member's appointment.
This means Ms O'Neill will miss the first sitting week of the new federal parliament when it returns on November 12.
Mr Carr resigned from both his current term and the following six-year term he was re-elected for at the September 7 election.
A spokesman for NSW Legislative Council leader of the government Michael Gallacher told AAP the state government had obtained legal advice for the unprecedented situation.
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"At this stage I believe we will only be filling this vacancy for the remainder of Mr Carr's term," he said.
"If the Labor party resolve that Deb O'Neill be the candidate for the second term, we'll need to have a second joint sitting."
He said legally the two terms could not be signed off on in one joint sitting or a concurrent joint sitting.
Ms O'Neill lost her NSW Central Coast lower house seat of Robertson at the election and is now moving to the upper house.
