After his aircraft, carrying five passengers, was attacked after landing in the isolated mountain province of Nduga, Mr. Mehrtens, 37, was taken into custody.
The West Papua National Liberation Army (TPNPB), his captors, have informed BBC Indonesian that he is "safe."
But they assert that they won't free him until West Papua's independence is acknowledged.
According to Sebby Sambom, a TPNPB official, the five additional passengers—including a child—had been freed since they were local Papuans.
Authorities in Indonesia claim to be sending out a team for search and rescue. However, police stated that because the isolated location can only be reached by air, it was logistically challenging.
New Zealand's prime minister, Chris Hipkins, stated that the embassy there was "working on the case" at the time. Prior to now, the foreign ministry had stated that it was "fully aware" of the circumstance.
The Susi Air-owned small passenger aircraft had left Central Papua's Mozes Kilangin airfield early on Tuesday with the intention of returning a few hours later after dropping off the passengers in Nduga.
During political negotiations, Mr. Mehrtens would be used as "leverage," a TPNPB spokesman told BBC Indonesian. He had been sent to the group's stronghold district in a far-off location.
