"At the population level, we're looking at a problem that affects about one in three women in their lifetime," she says.
"There's a high crossover between violence that happens against women in partnerships and child abuse and neglect."
Fanslow says that the lack of attention paid to this issue can be attributed to the sense of "despair and inevitability around the problems of family and sexual violence".
The reality is that family violence doesn't have to be inevitable.
"The evidence actually points in the other direction. It shows that family and sexual violence are preventable problems, but we actually need to work on investing in those solutions. If we were to have a more sustained focus on problems of intimate partner violence and child abuse and neglect, we could get somewhere."
Because domestic violence is so rampant, the number of cases that actually get reported to the police is often only a fraction of what is really going on behind the scenes.
Fanslow says that some estimates suggest that the percentage of cases that actually make it to the police possibly represent only 20 per cent of the incidents that are actually happening in the community.
Domestic violence can take many forms, many of which don't involve physical violence.
"It can be physical violence, it can be sexual and it can also be psychological, which includes humiliation and threats. You also have economic abuse and a whole range of controlling behaviours, which include limiting contact with family and friends. Part of the problem is that it's quite insidious and can take a whole lot of different shapes."
The Government has taken steps to address the widespread nature of domestic violence, but Fanslow says still more has to be done.
"This Government has invested quite a bit in the area of addressing family and sexual violence. We saw something like $114 million invested into this area in the last budget and over $100 million in the Budget the year before. This really signals the Government's priority around it. They've also been putting together a new national strategy focused on prevention of family and sexual violence and have been working on building stronger infrastructure around it."
But all this investment comes off the back of years of under-investment.
"Part of the challenge is that a lot of this money is kind of a remedial prop-up for services that have been underfunded for a lot of years. It's important, but we have some way to go."
Listen to today's episode of The Front Page to hear more on why we're doing so bad when it comes to domestic violence, and what could be done better to protect and prevent it.
• The Front Page is a daily news podcast from the New Zealand Herald, available to listen to every weekday from 5am.
• You can follow the podcast at iHeartRadio, Apple Podcasts, Spotify, or wherever you get your podcasts.