Airlines do not have to fit flight deck barriers on small commuter planes.
A review of aviation security was carried out after a 19-seater plane from Blenheim to Christchurch was hijacked in 2008 by a vineyard worker who stabbed both pilots.
Transport Minister Steven Joyce says installing cockpit doors would be too expensive and some costs would be passed on to passengers, by about $2 per seat.
He says officials were unable to find examples where other countries have bolstered flight deck security on small commuter planes.
However, barriers are being installed on aircraft with 30 or more seats.
Mr Joyce says new security measures at provincial airports will boost monitoring of passengers before they board planes with no cockpit barriers.