Irwin was best-known for his work with crocodiles
Family and friends of the man known as the Crocodile Hunter reportedly joined the low-key ceremony in Caloundra on the Sunshine Coast.
It is believed that afterwards Mr Irwin was to be buried at Australia Zoo, the Queensland zoo dedicated to Australian fauna owned by Mr Irwin's family.
Details of the funeral service are expected to be made public on Monday.
"(The) service was a service for family and good friends, people who were close to Steve in recent years," the Brisbane-based Sunday Mail newspaper quoted an unidentified family friend as saying.
"The council gave the family permission to bury Steve at the zoo and we think they're going to erect a monument there so visitors can continue to pay their respects," the person added.
The Australian government had offered a state funeral for the much-loved TV presenter who died in a stingray attack on Monday, but the family chose a small, private ceremony instead.
"He's an ordinary guy, and he wants to be remembered as an ordinary bloke," Steve Irwin's father Bob said.
Mr Stainton said Mr Irwin's wife, Terri, and the couple's two children - Bindi, eight, and Bob, two - were coping "quite well".
"Terri is very, very strong," he said. "She's having a lot of sad moments obviously, but she's putting on a brave face for the kids' sake."
The 44-year-old naturalist died after being struck in the chest by the stingray's barb while filming a documentary on Queensland's Great Barrier Reef.
Donations have flooded into Mr Irwin's charity Wildlife Warriors, while flowers, cards and trademark khaki shirts have been left at the Australian Zoo he ran.