Prime Minister Malcolm Turnbull has attacked Kristina Keneally's position on asylum seekers in his first appearance in the Bennelong byelection.

Labor's star candidate has been surging in the polls ahead of the byelection, which was called after incumbent Liberal John Alexander was forced to resign after being caught out in the citizenship scandal.

Mr Turnbull joined the campaign on Sunday and told voters to consider the former NSW premier's position on immigration as they prepared to vote on December 16.

"Kristina Keneally wants us to bring all of those asylum seekers from Manus [Island] to Australia," Mr Turnbull said in Sydney on Sunday.