(CNN) Former Australian Prime Minister Tony Abbott was allegedly assaulted while campaigning for a "no" vote in an increasingly divisive national survey over whether to allow same-sex marriage.

Abbott was headbutted Thursday by a man wearing a "yes" badge in Hobart, Tasmania, the former leader told CNN affiliate Sky News.

Australians are being asked to cast postal votes on whether the law should be changed to allow same-sex marriage. They have until November 7 to return their forms.

"A fellow sung out at me, 'hey Tony,' I turned around, there was a chap wearing a Vote Yes badge," Abbott told Melbourne radio station 3AW.

"I went over to shake his hand then he head-butted me," he added, saying the assault left him with a "very slightly swollen lip."