In an escalating conflict, the CFA has vowed to take action against any "unauthorised fundraising activities" used to support a campaign by Volunteer Fire Brigades Victoria against a controversial industrial deal with the firefighters union.

The volunteers' campaign – which included a costly Supreme Court battle – followed claims the agreement would have undermined their role and given the union excessive powers to interfere in CFA management decisions.

In a letter sent to all brigades on Friday, CFA chief executive Frances Diver said "all instances" of unauthorised fundraising would be investigated, warning individuals could face penalties.

"CFA is aware that members and brigades may have undertaken unauthorised fundraising activities to support the VFBV," the letter reads.

"This is a serious matter and may open individuals up to penalties under the CFA Act and the Public Administration Act."

The CFA relies on donations and fundraising activities to help pay for equipment and operational costs. It does this using a special public trust fund, set up to allow people to make tax-deductible donations.

Strict rules govern the fund, including a requirement that fundraising activities must be authorised by the CFA with stringent rules about how the money is raised and used.

In a separate letter to Volunteer Fire Brigades Victoria chief executive Andrew Ford, also seen by Fairfax Media, Ms Diver warns "moneys from the trust may be being utilised for improper purposes".

The letter notes that the trust has seven trustees, five of them nominated by the VFBV.

The investigation will further stoke tensions between the Andrews government and volunteer firefighters. It follows a long-running dispute over the union deal that has led to a string of resignations and the sacking the entire CFA board.

Premier Daniel Andrews controversially backed the union deal triggering the resignation of minister Jane Garrett and plunging his government into internal crisis. The scandal also resulted in the sacking of the CFA board and the resignations of Lucinda Nolan as CFA chief executive and Joe Buffone as chief fire officer.

The letter to Mr Ford says that by agreement CFA brigades are able to use CFA logo to raise money for operational uses, such as new equipment.

But Ms Diver says the CFA has not received any request to fund-raise to support VFBV activities.

The volunteers said they were entitled to fund-raise and the majority of donations came directly from individuals, not from brigade fundraising. 

The Andrews government said it was a matter for the CFA.