Opposition Leader Bill Shorten has been forced to deny his party is disunited on the issue of boat arrivals for the second day in a row after a photo emerged of the Labor candidate for a marginal seat at an asylum seeker protest.

The photo of Cathy O'Toole, distributed widely by Coalition strategists, showed her protesting outside the office of sitting Liberal National Party MP Ewen Jones holding a sign saying "let them stay".

This followed comments on Monday by Labor's candidate for Melbourne, Sophie Ismail, saying boat turnbacks should be off the table even though they are part of official Labor policy.