Kevin Rudd's sister has quit the Australian Labor Party, saying she cannot back a party that supports "homosexuals marrying".

Loree Rudd, 61, said she sent the letter to her Nambour branch last week after a policy shift at the ALP National Conference on December 3.

Loree Rudd is quitting the ALP over its decision to change its stance on gay marriage. Credit:Brett Wortman/Sunshine Coast Daily

Delegates voted to change the party policy from opposing to supporting gay marriage, with Labor MPs and senators granted a conscience vote when a bill is considered in parliament.

"I knew I didn't want to stay in a party if that was their stand," she said.