When you sentence a person to prison, you are sentencing their families as well, says a woman whose life went into a ''tailspin'' after her partner was imprisoned.

The Hastings resident, who is using the pseudonym Helen to protect her former stepchildren, is speaking out to draw attention to the ''second sentence'' families face.

''We knew he might go to jail for the assault, but thought we would have time to prepare. Instead he was just suddenly gone.

''I was in a tailspin. It was suffocating. I was trapped.''

Her comments are backed up by University of Auckland masters student Ivana Mlinac, who has just released her thesis on how the criminal justice system affects offenders' families.

Ms Mlinac found children of prisoners struggle with anxiety, bullying at school and grief over losing a parent, while the family suffers from the loss of income.

Those children had higher dropout rates for school and employment, over-representation in the criminal justice system, and there were detrimental effects to their health and wellbeing.

An estimated 23,000 children in New Zealand are affected by a parent in prison.

''It's a huge, huge list of what goes on in a child's life,'' Ms Mlinac said.

Helen was living with her fiance, his two daughters and her daughter, all aged under 8, when he was sentenced to nine months in prison for assault in 2010.

The household income had gone down, but expenses went up.

Helen was able to go on a benefit for herself and her daughter but could not get any financial assistance for her partner's children for six weeks. She said Work and Income eventually allowed her to add the other children to her benefit and backpaid her.

She also supported her partner while he was in jail. She sent him $50 a week for phone cards and cigarettes. It used to cost $1/minute for him to call home.

But the hardest part was helping the children cope.

''It was as birthdays came and he wasn't there, over Christmas as well. They had to do all of that without him.''

They would speak to him on the phone but his phone card would often run out a few minutes into the conversation. She tried to take them for visits when they could get transport.

Helen said more contact would have helped.