Cracks are emerging in the campaign attacking the Andrews government's deal with the firefighters' union, as more volunteers speak out against "misinformation" that is stirring "animosity, anger and uncertainty".

Premier Daniel Andrews' decision to back a union deal has already led to minister Jane Garrett quitting and fuelled widespread community anger, with rallies across the state against the Premier's intervention.

But in a scathing letter, obtained by Fairfax Media, volunteer firefighters at Greenvale CFA in Melbourne's north have condemned the Volunteer Fire Brigades Victoria association for highly public and inflammatory comments about the impact of the union deal.

The association has been leading the charge against the agreement, saying it amounts to a takeover of the largely volunteer firefighting force by giving the United Firefighters' Union sweeping powers to interfere with management and operations.