An Illawarra man has been jailed for 14 years for the horrific sexual abuse of his two young stepdaughters over more than half a decade.

The man, who cannot be named to protect the identity of the victims, repeatedly molested both girls while living with them and their mother at a property south of Wollongong between 1998 and 2003.

The children, aged five and 10 when the assaults began, were often forced to perform sexual acts on the man while on other occasions he rubbed himself against them.

What the older victim told the court

The eldest girl was raped several times, once after she discovered the man moments away from abusing her sister and came to the girl's rescue, and another time while she was in the lounge room watching television.

Presiding judge Paul Conlon said the man's "predatory conduct" began almost immediately after he moved into the house and involved an escalating pattern of abuse lasting six years.

"He has exploited these vulnerable young children in the worst possible way," Judge Conlon said in Wollongong District Court on Thursday.

"He was in the position of a stepfather and accordingly the offences involved gross breaches of trust.

"He left behind two devastated and broken young lives."

The court heard that at one stage the girls wrote a letter to their mother detailing the assaults; however, she did not believe their story and forced them to apologise to the man.

When they did, he told them it wouldn't happen again. However, he started abusing them again just weeks later and only stopped when he and the girls' mother separated in 2003.

The women eventually reported him to police in 2012. The man was arrested a year ago and pleaded guilty in November to two charges of persistent sexual abuse of a child.

Addressing the now adult victims, both of whom were in court to hear the judgment, Judge Conlon said he recognised their suffering.

"As young children, you deserved the love, care and protection of the adult male in your home," he said. "Instead you received betrayal."

However, he also acknowledged the great strength the girls had shown in attempting to mend their lives.

The court heard that the oldest victim, now aged 24, had had a troubled teenage life, often resorting to self harm, drugs and alcohol, but had turned her life around after giving birth to her daughter and turning to Christianity.

In a victim impact statement read to the court, the woman said she had forgiven her stepfather for the abuse, prompted by his confession to the crime and apology to her.

"In this moment, that 10-tonne weight I had felt my entire life lifted and I felt such peace," she said.

"I'm no longer the sad, broken girl ... I accept your apology, I forgive you," she said.

However, the girl's younger sister expressed a very different attitude towards her stepfather.

"I have tried to find a place in my heart and mind where I can forgive you but I'm sorry, I just can't forgive you; you're the man who changed so much and took so much away from me as a child," the now 21-year-old said in her court statement.

The man was given a minimum jail sentence of 10 years, meaning he will be eligible to apply for parole in 2023.