The University of Otago study involved 6670 new married, single and co-habitating parents who answered questionnaires about aspects of their health in 2004, 2006 and 2008, after having their first and subsequent children.
Comparing their responses, researchers discovered becoming a parent for the first time was good news for mothers and fathers, who experienced a small improvement in their mental health.
But this was not the case with subsequent children.
This result differs from overseas research, which showed major differences between the results for women and men.
The findings were surprising, university research fellow Dr Kristie Carter said.
"I thought it was interesting. It's possibly due to the fact that we have a larger sample size. It's not all negative and even if it does take a year for your mental health to get better, that's a good thing."
The study took into account partner status, employment, deprivation and household income, but further work was needed to examine changes in socio-economic factors when a child comes along and how these affect new parents, said co-author, research fellow Sarah Mckenzie.
While some first-time parents may experience improved mental health, this was not true for everyone and women are most vulnerable to mental illness after childbirth because of the "huge hormonal changes they experience", said Dr Alison Wenzural, Waitemata District Health Board Maternal Mental Health Service consultant psychiatrist.
More than 10 per cent of mothers suffered from depressive illness after childbirth and about 3 per cent of cases were moderate to severe.
Those affected had a 40 per cent chance of recurrence with future pregnancies, said Dr Wenzural.
"So as well as celebrating the positive experience of so many parents, it is also important to be mindful of the potential risks to women's mental health, as well as the positives the study identifies," she said.
The university study was published in the international Journal of Epidemiology and Community Health.
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There's no question having daughter Arielle has positively effected her mental health, says first-time mum Catherine Sutton.
The 30-year-old from Auckland agreed with the study's findings that being a new mum or dad could make you feel happier and less anxious?
"Every day you wake up and know you've got this little human who's totally devoted and dependent on you.
"There's nothing to match the absolute love and devotion you feel for your child and the happiness they bring to you each day, which has to go someway into helping your mental health and well being," she said.
However, being a first-time parent could also be stressful, and episodes of "baby brain" were common for the new mum.
Life had changed completely since she and husband Brent became parents five-and-a-half weeks ago and she agreed with experts that knowing what to expect with subsequent children could change the impact on parents' mental health.
"Being first time parents is a big guessing game. For second-time parents you know a lot more."
