The Royal Commission, appointed by former Prime Minister Jacinda Ardern in 2018, is investigating wrongdoings in state and faith-based care between 1950 and 1999.
It was originally granted an extension to the middle of this year, but inquiry chairwoman Coral Shaw requested another pushback due to the sheer weight of material to assess.
"The scale of abuse is beyond what anyone had ever imagined at the start of this inquiry," Ms Shaw said.
"More time ensures our final report and recommendations are impactful and meaningful for survivors."
On Wednesday, the current minister Barbara Edmonds said the new extension had been granted to deliver on a promise.
"We made a commitment that every survivor who wanted to be heard would have to opportunity to do so," she said.
"This timeframe extension reflects the huge amount of evidence received and the need for every voice to be included."
The final report will now be delivered to government by 28 March 2024.
A spokeswoman for the Royal Commission said the extension would not increase its budget, already the most expensive probe in NZ history.
In 2021, Radio NZ reported it had blown its initial budget of $NZ56 million ($A52 million).
In that year's budget, Finance Minister Grant Robertson granted the Royal Commission another $NZ100 million ($A93 million) to complete its work.
Alongside the commission's work, the government is also designing a survivor-led independent redress system for historic abuse in care, appointing Annabel Ahuriri-Driscoll and Ruth Jones as co-chairs for that work.
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