Labour leader Jacinda Ardern has offered to support researchers running a landmark longitudinal study, whose cohort was slashed by two thirds after a Government funding cut.

The Growing Up in New Zealand study, led by Auckland University and touted as the country's "contemporary longitudinal study", has been tracking the development of thousands of Kiwi children born between April 2009 and March 2010 from before birth until they are young adults.

The study was designed to provide unique information about what shapes children's early development and how interventions might be targeted at the earliest opportunity to give every New Zealand child the best start in life.

Its latest finding, released this week, suggested a genetic variant could interact with other factors to contribute to childhood obesity, particularly in young Maori and Pacific boys.

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The study's cohort for the next eight years was reduced from 7000 children to 2000 last October, after a contract change between the university's commercial arm, UniServices, and Superu, formerly the Families Commission.

The cut drew heavy criticism at the time from Ardern, as Labour's social development spokeswoman, and the New Zealand Association of Scientists.

Auckland-based paediatrician Dr Katie Tuck has since launched an Actionstation petition calling on the Government to restore funding to the previous level, saying $1.4m was "urgently needed" to ensure all of the original children and families were included in the data wave.

As of this afternoon, the petition was close to gathering a goal of 6000 signatures.

The leaders of the study, who are seeking funding themselves to include the full cohort, have declined to discuss the petition but stressed the campaign was organised without their involvement.

Speaking to media today, Ardern said her party had questioned "the ethics and scientific rigour" of cutting a longitudinal study part-way through.

"What we will do in office is sit down with those working on the study to make sure that whatever changes are made uphold the ethics and rigour that a study like that needs," she said.

"Changes have been made mid-stream - we've got to be careful that we don't disrupt the study again."

The Green Party's Marama Davidson has also tweeted "We need to save it. Particularly for its Maori, Pacific and Asian sampling work. Groundbreaking and otherwise scarce in other research."

Tuck, whose daughter is in the study, said the cut had been "so disappointing".

"We have invested so much time in this study over the past eight years and for the Government to cut funding when the children are so young seems so short-sighted."

The eight-year data collection wave was especially significant because the children had now started school - and this was the first time children will answer the questionnaires for themselves.

"The 8-year-old survey is the first time the child's voice is heard," said Dr Mae David, an Auckland Maori GP whose 8-year-old daughter is in the study.

"[The children are] asked about their favourite foods and activities and most importantly about their mood."

Questions about children's mood were particularly important given Unicef's recent finding that New Zealand has by far the highest rates of youth suicide in the OECD.

"Growing Up is a once-in-a-generation opportunity for us to find out what is driving these terrible statistics," Tuck said.

She was concerned the Government's new social investment approach might mean such child health studies weren't seen to be as important.

"You can't get the same sort of rich information from administrative datasets which don't include people's voices, let alone children's voices.

"These datasets don't have the biological samples that allow genetic studies either."

Tuck said the $1.4m sum was "very small" when compared with the tens of millions already invested.

In a statement to the Herald, the study leaders said: "At this time core Government funding is available for Growing Up in New Zealand to engage a subset of the cohort to participate in the eight-year data collection wave.

"Together with the University of Auckland the Growing Up in New Zealand team is seeking additional funding that will allow them to engage with the full cohort going forward."

Social Development Minister Anne Tolley said the reduction had been an "operational decision" made by Superu, which the Government supported.

Tolley said the negotiated four-year contract was for $11.788 million, which she considered a "significant investment", and there was also a contestable research fund of $750,000 a year that researchers could bid into.

"Statistics experts were consulted by both Superu and UniServices to finalise the sample number of 2000 children, which is larger than similar highly internationally regarded New Zealand studies," Tolley said.

She said that, since the study began, more sources of longitudinal data had become available, "and how we think about longitudinal studies has evolved".

"A smaller sample size does not reduce the value of the study.

"All key population groups of interest will be retained, in particular, Maori, Pasifika and Asian ethnic groups.

"Superu advise me they are confident this sample size is robust and will enable good quality data to be collected."