I'm going to go ahead and post "Airscream" by John Bruce as the
official answer here.
I have located book descriptions at Abebooks.com that confirm all of
the essential elements described in the question (i.e., a mid-air
collision in New Zealand between a private plane and a passenger
aircraft; airport airspace; and a woman, presumably the pilot's wife,
thrust into a courtroom battle).
"A mid-air collision over Wellington between a large passenger jet and
a private plane is the basic plot of this all too plausible novel by a
new writer of great promise. Professional aircrew, ground control, an
expert layman and the complex technology of modern air travel all play
a part in an enthralling sequence of events which leads to an
inevitable conclusion.a screaming catastrophe. From the twisted
wreckage emerges a tangle of maimed bodies, minds and lives; order has
to be re-created by the available processes of civilized society. A
lone woman whose sole purpose is to see justice done is thrust into a
battle against the overwhelming complexities of big business and
government whose aim is to protect their vested interests and position
with all the might and power that they can wield. Legal tangles,
court-room statesmanship, political intrigue, the human havoc and a
shattering climax make the aftermath of the disaster as exciting and
compelling as the event itself."
"DC8 Paranational Zulu Delta is on the last leg of a routine flight
into Wellington: the captain has already started the procedure for
descent; Frank Newton, a New Zealand vet in a private Cessna, has
taken off and is climbing steadily; in cloud, he is flying on
instruments under the guidance of Wellington Ground Control; at 4.46
they will collide; within two minutes, 168 men, women and children
will lose their lives. saga of mid-air collision, tangled emotional
lives, political chicanery and a tough legal battle"