The Save Cadbury Community Action Group is hoping for a large turnout to the Octagon at 11am on Saturday to call on Mondelez International to ditch its proposal to close the factory.
Mr Cull confirmed yesterday he intended to accept an invitation from organisers to speak at the rally, but he declined to comment further.
Mondelez has proposed closing the factory and keeping the tourist attraction Cadbury World.
Cr Damian Newell, who is attending the rally, spoke in favour of a strong public showing at the event.
Councillors had been given no instruction about what they could say in public, he said.
''We've certainly had no briefing from council about what we can and can't say, and what we can and can't do.
''I think there is a real fear that we could lose Cadbury World as well. Who knows, that could be the next part of this.
''[The rally] is a respectful way of telling [Mondelez] the level of anger and hurt that this sort of decision makes.
Other councillors planning to attend are: Rachel Elder, Conrad Stedman, Jim O'Malley, and Christine Garey.
Cr Lee Vandervis said there was no point to the rally, because the factory closure was a ''foregone conclusion''.
Cr Doug Hall would not attend, and Crs Mike Lord, Andrew Whiley, Marie Laufiso, and Chris Staynes would be out of town.
Cr Aaron Hawkins, Cr Kate Wilson, and Cr David Benson-Pope could not be contacted.
Action group chairman Don Pryde said the event had been organised at short notice, because it needed to happen before Mondelez's final decision.
Mr Pryde said a public show of support could play a part in Mondelez's decision-making process.
''Cheeky'' placards were fine, but they must be respectful, or people would be asked not to show them.''
Etu national industry strategy director Neville Donaldson said a good turnout could help the union's bid to keep the plant open.
''We have been in talks with Mondelez who are supposed to genuinely consult over their proposal.
''However, to date they have shown little interest in working through the issues or any concerns about the effect on staff.
''A strong show of opposition on Saturday to its plans for Cadbury may persuade them it's time to take the consultation process seriously and reconsider,'' Mr Donaldson said.
Mondelez New Zealand country head James Kane acknowledged the strength of feeling around the proposed closure.
''We are committed to working through consultation with our people and their representatives to ensure we consider all possible options,'' he said.
''This is our focus at the moment, and we are in constant communication with the union and our people to progress this work.
''We have agreed with the union that while we are focused on communicating directly with them and our people, we won't be providing public comment at this stage.''
Mondelez confirmed Cadbury workers are free to join in.
