The four birds were shot during a pukeko culling operation on the island in August involving members of the local deerstalkers association.
DoC's deputy director-general operations, Mike Slater, said all the cullers involved were fully briefed on how to identify their targets and directed to only shoot birds on the wing.
Mr Slater said a detailed forensic investigation, including analysis of pellets found in the birds, confirmed the shots came from the guns of two members of the culling party.
He said the men involved were experienced game bird and shotgun hunters. They had been specifically briefed on the differences between pukeko and takahe and clearly instructed to only shoot birds on the wing.
"By failing to identify their target, they have breached a fundamental principle of the firearms code. As a result, we are referring the issue to the police to assess whether these men should continue to hold firearms licences."
Mr Slater said controlling pukeko was an ongoing issue for DoC and other conservation groups.
He said failure to identify the target was the cause of the takahe deaths on Motutapu but DoC had also developed new national guidelines to standardise planning for future culling operations as a result of this incident.
There is a new directive that all culling operations where threatened species are present must be approved at national administration level.
The Motutapu Restoration Trust said members were satisfied that DoC had reviewed its internal processes and was taking the necessary steps to stop a repeat of the bird killings.
"We believe Motutapu is still a suitable island habitat for takahe and look forward to working with the Takahe Recovery Group to grow the population into the future," it said.
"We support research into the most effective techniques for managing pukeko, which are a widespread problem for many areas of NZ, where they threaten other, rare species, and cause significant damage to restoration plantings."
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