The Defence Force told the Australian military it had contaminated water at its bases months before it alerted the New Zealand public for the first time, emails released under the Official Information Act show.
The newly released emails also show Defence gave the Australians a heads up, including about investigations at Devonport, before it told the government.
Defence told the New Zealand public about the contamination with long-lasting and damaging PFAS chemicals in firefighting foam, in early December last year.
It told the Defence Minister last August.
But two months before that, in June 2017, its Environmental Services unit emailed the Australian Defence Force saying it was "about time" they conferred about the contamination.
"As you know, we have been undertaking detailed/intrusive investigations at our Air Force (and also Navy) sites and have recently received some interesting results."
The Australians replied four days later that they would be "very interested" to see those results.
There is no email record whether that happened.
The contamination at the Navy base, at Devonport in Auckland, was not made public in this country until nine months later, in February 2018, and then only after media inquiries.
On August 3, 2017, the military briefed the Defence Minister Mark Mitchell, and a week later emailed their Australian counterparts: "Detailed site investigations at one particular site has shown elevated levels of PFAS in both surface water and groundwater at the base boundary.
"Since Australia is leaps and bounds ahead of NZ we are currently trying to gauge the similarities."
In the emails, the New Zealanders asked about quantities and use of foam, and also said they wanted the Australian military's advice, including "urgent assistance" about communicating with the public.
"We believe that we are now in a position to liaise with you and your team and draw on some of your knowledge and experience in relation to personnel comms (internal and external), risk management, recommended actions and other strategies used by Australian Defence throughout their investigations," said an email in June 2017.
The Australians obliged, with tips such as for running a national hotline.
However, in October 2017, the Australian Defence Department's deputy secretary Steve Grzeskowiak made the public admission it was wrong to have waited till 2015 to tell the public across the Tasman about the contamination.
"I think if we had our time again, should we have told the community back in 2012, from the middle of 2012? We probably should."
The Official Information Act response to RNZ suggests there was no trans-Tasman military correspondence about PFAS before April 2017, though 2016 was not included in the request.
RNZ asked for correspondence between 2002 and 2015, but the emails released date only from April 2017.
In August 2017, Defence told Mr Mitchell: "The Australian Defence Force has been facing similar issues in recent years and the New Zealand Defence Force has engaged with them to benefit from their experience."
Other documents show some Defence staff did not know in 2015 that one of the PFAS compounds, called PFOS, was banned, though it had been made illegal years before.
The OIA emails show Defence first spotted a media story about the chemicals only in July 2017; this was almost two years after stories began running in the Australian media.
Its briefing to the government said that since the early 2000s, PFAS compounds "have been increasingly recognised as potentially harmful" and "internationally, a precautionary approach to human exposure is being adopted".
The longer a person, plant or animal is exposed to the chemical, the more they build up inside them.
"Little was known in New Zealand about the issue of PFAS compounds in the early stages of our investigations," Defence said last night in a statement. It declined to be interviewed.
"It was an emerging contaminant. The Australian Defence Force is more advanced in their investigations so it is therefore prudent for NZDF to seek advice from our Australian counterparts during our preliminary investigations and as we continue to work through this issue."
The Defence Force has previously said it did not alert the public in 2015, when its tests first showed contamination at Ohakea airbase, because there were no health safety level guidelines for the two most high-profile PFAS compounds, PFOS and PFOA.
New Zealand adopted Australia's guidelines in April 2017.
Australian authorities have faced similar questions about the lag in their response.
A 2016 report to the federal government about New South Wales' response said that "a lack of guidelines may have meant that sites potentially contaminated with PFOS/PFOA have not been notified because there are no national trigger values".
The NSW Environment Protection Authority "could have acted earlier in developing or adopting interim guidelines. Capability for PFOS analysis was available in Australia since at least 2005. Therefore this was not a limiting factor to developing ...guidelines".
The Australian Defence Force was warned in 2003 that PFASs were persistent, toxic and accumulated in the body.
An Australian university study in 2013 concluded that PFASs have "both acute and chronic impact" upon land and water animals and plants, and people.
As for switching to less damaging foam, the emails released under the OIA show Defence started to ask the Australians about this in August last year.
This was a decade after the world's eight largest makers of the chemicals agreed to get rid of PFOA by the end of 2015.
The Defence Force had plenty of environmental advisers during this time: In late 2016 it awarded a seven-year contract covering 19 companies to be on an environmental consultancy services panel.