There is no specific ratio prescribed towards fire fighters – your risk assessment should determine this.
Section 8 of the Occupational Health & Safety Act (OHS Act), specifies that the employer shall provide and maintain; as far as is reasonably practicable; a working environment that is safe and without risk to the health of his employees. By law, employers must identify, evaluate, remove or mitigate work related hazards and risks. This includes planning and preparation for the inevitable, like a fire or evacuation.
Section 13 of the OHS Act stipulates that all employees should be informed of work related hazards, including fire, as well as the precautionary measures taken to protect them.
Note: Important – your fire fighter and first aider may not be the same person. A conflict of interest can occur in case of an emergency – “will I treat the injured persons as first aider or will I act as a fire fighter?”