South Australian Rex Patrick is now an independent senator after resigning from Centre Alliance.
Announcing the split, Senator Patrick said strong independent representation would be important for SA as it recovered economically from the coronavirus pandemic.
"I have concluded that in seeking to continue to represent SA, and to ensure that our state's Senate representation includes a voice from the political centre, the best way forward for me is to strike an independent path," he said on Monday.
"The Senate will be dealing with many challenging legislative issues through the rest of this parliamentary term, many of which will be vitally important to South Australians.
"I will use the freedom that comes with being an independent to press hard to win the best outcomes for SA."
Senator Patrick entered parliament in 2017 to replace Nick Xenophon for the centrist minor party, formerly known as Nick Xenophon Team.
Senator Stirling Griff and MP for Mayo Rebekha Sharkie remain as members of Centre Alliance.
Australian Associated Press