Rebecca Gibney apologises for ancestor's Parihaka role

Rebecca Gibney.

Packed to the Rafters star Rebecca Gibney has taken a walk down a very dark memory lane and opened some deep emotional wounds on the series Who Do You Think You Are?

The Australian-based Gibney returned to New Zealand on the SBS series and confronts her great grandfather's role in an infamous war, learns more about her abusive father's past and meets a forgotten relative.

None of it was easy for the actor to emotionally digest but she felt it was something she had to do for her 79-year-old mother Shirley.

Gibney said, until now, she had shunned the past and what she discovered for the show hurt. But it also started a healing process and has improved her relationship with her extended family.

"It is a mystery tour and I didn't really want to know about my past because I came from a very dysfunctional family," Gibney told AAP.

"One of the reasons I wanted to do the programme is I didn't know anything about my grandfather or beyond that because I didn't want to, because he caused a lot of trauma to our family.

"I had shut the door on that."

But it was a journey she finally felt she could take "for my mother because she wanted to know why he was the way he was."

Gibney finds out her great great grandfather James Way Jnr fathered 16 children but he also was involved in an unsavoury chapter of New Zealand history.

He was part of a military invasion of the peaceful Maori settlement of Parihaka in Taranaki in the late 1800s.

She visits the Maori descendants of that invasion and makes an emotional apology on behalf of her family.

"It was incredibly confronting and on the spot I had to come up with a response which was really difficult," she said.

"It had to be treated with incredible care and I was thrown and was terribly emotional.

"The impact on me was quite profound and still has an impact on me."

Gibney also discovers her grandmother was 19-years-old and pregnant when she arrived in New Zealand, from London, in 1913 and then surrendered her daughter, Kathleen, to foster care.

A highlight of her trip was meeting Kathleen's surviving daughter in Auckland.

"It's been a healing time for the family and opened the door on family relationships," she said.