Alexandra firefighters quickly extinguished the blaze in the back room of St John The Baptist Catholic Church after being alerted at 3.20pm yesterday.
The fire officer in charge at the scene, Derek Shaw, of Alexandra, who is also a police detective, said a child had been referred to Youth Aid following the blaze.
The scene inside the room ''strongly indicated'' the fire was suspicious. A box of matches was found inside.
There was damage estimated about $6000 to the floor, walls, curtains and a window, he said.
''That's not so much our point. The point is that it could have been substantially worse.
''Arson is repugnant at the best of times, but it's a bit disturbing that it's a volunteer organisation like a church,'' Det Shaw said.
Emergency services were alerted to the incident by Firefighter Shayne Ryan, who saw the smoke while attending a parent-teacher interview at St Gerard's School next door to the church, he said.
School principal Trina Eastwood said three pupils also noticed the fire.
''Three boys together saw smoke coming out of the door. The eldest opened the door and black smoke came out and they called mum right away, who called the fire brigade.''
Det Shaw thanked the community for their assistance.
