The father of two young boys allegedly murdered by their mother in the Blue Mountains has paid tribute to his "sweet" and "caring" sons at a funeral service attended by hundreds of friends and family. 

Russell and Ben Smith, aged nine and 11, were found dead in their Faulconbridge home by their father earlier this month.

Their mother, Patricia Smith, has since been charged with two counts of domestic violence-related murder.

Nick Smith said his children were passionate about fishing, family and music.

"We used to go fishing in lakes and streams and even the ocean," he said.

"Fishing was a happy place.

"Crazy frog and your gummy bear song you played over and over and that broke grandad on a couple of car trips."

He said Russell was "outgoing, kind, determined, caring and thoughtful, adventurous and just a heart of gold, who no matter what wanted to help". 

Mr Smith described his other son Ben as a "sweet boy".

"The one name you gave yourself was Ben Jelly, and it stuck."

He said he would miss his children "so much". 

"You had such beautiful little lives, you were such wonderful little boys. 

"We were the best friend club crew, and we had so much more adventure to come." 

More than 700 family and friends gathered for the funeral in Springwood on the outskirts of Sydney. 

Scores of school-aged children wearing Penrith Panthers jerseys and the boys' favourite colours of orange, red and blue arrived for the Catholic Service on Wednesday, which was held at St Thomas Aquinas Church on the outskirts of Sydney. 

Joyful music could be heard from outside the service, which was closed to the public.

The boys' school Our Lady of The Nativity Primary School gave students the day off to attend the funeral.