Mehrtens, who was a pilot for Susi Air, was taken hostage by TPNPB fighters in February this year, shortly after landing at a remote airstrip.
A new video circulating on social media shows Mehrtens surrounded by armed men. The video said Mehrtens had been given two months to live before he will be shot dead.
RNZ has been unable to verify the footage or when it was filmed.
However, West Papua spokesperson Akouboo Amatus Douw said the video dated back to May or June and Mehrtens remained in good health.
"I just received a message from our teamwork [sic] just a few minutes ago and they said that he is still alive, he is very healthy at [the] moment," Douw said on Wednesday.
"We the liberation fighters gave him very good hospitality which is if he needs some medication or water or food or whatever, they [will] always provide since they captured him in February.
"We have our own humanitarian team inside the TPNPB so that team looks after him. His wellbeing and welfare is a top priority."
The Ministry of Foreign Affairs (MFAT) was aware of the video, it said in a statement.
Efforts were ongoing to secure Mehrtens' safe release, it said, including working closely with Indonesian authorities and deploying New Zealand consular staff to do so.
The pilot's safety and wellbeing remained the top priority, and his family in Aotearoa and Indonesia were being supported, MFAT said.
Douw said he wanted Mehrtens to be released under peaceful negotiations, facilitated by a third party.
He said the TPNPB wanted to release Mehrtens "very soon" but it depended on the New Zealand and Indonesian governments.
"We are ready, we are happy, we hope [to release Mehrtens] anytime soon but it's depends on the Indonesian government and also New Zealand how they would like to go."
Douw said discussions were "deadlocked" with New Zealand and Indonesia so the TPNPB is now waiting for the new New Zealand government to be established.
"I hope that the New Zealand prime minister and foreign minister, they might understand about how West Papuans struggle."
Douw said he thought Mehrtens would remain safe.
Dr Socratez Yoman, the president of the fellowship of Baptist Churches in West Papua based in Jayapura – the largest city in the Indonesian province of Papua – said he believed Mehrtens would remain safe and not be killed.
"My people are not savages, they are not savage people, they have dignity, they respect, human dignity," Yoman said. Yoman said he hoped Mehrtens would be released soon.