The woman, who fought back tears as she read her statement in court, said she felt as though she had failed as a mother because she had not detected the "disgusting" offending.
"The only reason I'm here is because of my girls, because I love them with every ounce of my being and they need me," she said.
"You claim to have loved them, but you came in and destroyed our world with no thought for anything else but your own gratification."
Judge Michael Crosbie said some of the offending happened at the weekend, while the mother was working.
It happened in the man's home, in his car, at a motel and while on holiday too, the court heard.
The older girl was abused from the age of seven, while the younger was molested from when she was four years old.
When the victims tried to walk away he would tell them to stop, the court was told.
"She'd tell you it hurt and you'd tell her that it didn't," the judge said.
The man would buy the girls lollies and tell them not to disclose the lewd acts to their mother because it was their secret.
But the defendant's pattern of sexual offending was finally broken when he was caught by his wife in the midst of an indecent act with one of the girls.
The court was told she was out collecting firewood from the garage when she saw her partner in a compromising position.
The judge said it was a said indictment on society that such offending was so common - "that men like you prey on the vulnerable, that men like you clearly capitalise on the trust in a relationship and violently sexually assault children."
The victims' mother sobbed at the back of the court throughout. Judge Crosbie was full of praise for her brave statement.
"You should not blame yourself for this," he told her.
"Offending of this type is always covert . . . it's deliberately planned so no one would know. This offending does not make you a bad mum. I think from what I've read, you're an excellent mum."
Defence counsel Nathan Laws said his client accepted he had an attraction to pre-pubescent children and wanted to seek treatment to address it.
"He hates himself for what he did," the lawyer said. "He knew it was wrong but simply could not stop."
The defendant had made an offer of $10,000 as an emotional-harm payment to each victim, which Mr Laws accepted may come across as a "token" gesture.
It was the majority of his clients assets though, he said.
Judge Crosbie ordered $5000 reparation for each victim, but accepted it "doesn't even scratch the surface of what's been done".
The defendant will be automatically added to the Child Sex Offender Register.
