Immigration Minister Kris Faafoi said some visitors were keen to stay in New Zealand while the Covid-19 pandemic raged overseas.
The travel and business sectors has also lobbied for the extension because foreign visitors who are still here could help local economies.
As well as the automatic extension, the Government has agreed to temporarily waive a rule which states that visitor visa holders can only be in New Zealand for nine months out of an 18-month period.
"Most visa holders currently in New Zealand will have been here for more than nine months by now, which would have excluded them from applying for another visitor visa," said Faafoi.
"Waiving this nine months of 18 rule means that visa holders who are still here can apply for a further six-month visa."
Faafoi said the visitors whose visas are set to expire after March 31 won't be eligible for the automatic extension, they could apply for the secondary change.
Faafoi also sent a warning to those on visitor visas - that the Government won't guarantee any more extensions.
"While we have made these changes to help many visitors facing uncertain and worrying situations back in their home countries because of the Covid-19 pandemic, the Government wants to make clear that people who are here on visitor visas are here as guests for a limited time.
"We cannot guarantee further extensions or waivers when these changes expire," Faafoi said.
He said the Government expects all migrants in New Zealand to ensure they are here lawfully and are able to support themselves.
"Temporary migrants who are in New Zealand without the means to support themselves, or who become unlawful, are at risk of migrant exploitation or not being able to meet their basic needs.
"Migrants who stay here after their visas have expired also risk having their unlawful status count against them should they wish to travel to New Zealand or elsewhere in the world in the future," Faafoi warned.
