She said New Zealanders from around the country had expressed their condolences to those who had lost loved ones or had been affected by the fires.
"I extend mine, too," Ardern said.
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"It's simply devastating to see the scale of the loss."
Death toll now eight, many missing
The latest figure for the death toll stands at eight; while 17 people are missing in Victoria.
As at 11.45am (NZT), fire authorities in Australia said there were 127 fires burning and 67 had not been contained.
"Around 2100 firefighters are in the field," the New South Wales Rural Fire Service said on its official Twitter page.
The most devastated places are in Victoria and NSW, where a state of emergency has been declared.
Ardern said she had been in contact with Australian PM Scott Morrison to pass on New Zealand's support.
Before today's announcement, 157 New Zealand firefighters had been deployed to help with the fires across the Tasman.
The first group were sent over last October.
Ministry of Internal Affairs Tracey Martin said the Government was monitoring the situation in Australia closely and Fire and Emergency NZ was also keeping in close contact with fire authorities in Australia in case more support on the ground is needed.
"I thank them for their incredible efforts and wish them the best for the tough and complex work they are doing," Martin said.
