Aircraft were put at risk of ''conflict'', not advised of possible wake disturbance and sequenced too close together.
In the fourth incident, a pilot on their first solo flight was put on a head-on converging path with another plane and the aircraft came within 926m of each other.
''Following the fourth incident, the assessor stopped the assessment and took over as the aerodrome controller to resolve the situation,'' the report says.
Airways says the controller at the centre of the investigation has since passed their proficiency assessments, progressed their career and been posted to another tower.
The commission hired a clinical psychologist to analyse the actions of the controller, who has not been identified.
''The psychologist's assessment of the controller's actions stated in part [that] the controller appears to have experienced 'choking under pressure', a phenomenon defined as a critical deterioration in the execution of habitual processes as a result of elevation in anxiety under perceived pressure, leading to substandard performance."
Airways said that following any safety event, it was standard for a controller to be stood down for an appropriate period.
''This is both for operational safety reasons, but also for the wellbeing of the controller to ensure they are confident to return to work,'' an Airways spokeswoman said.
New measures have been brought in, including new training systems by Airways, new work protocols and a re-shaped control zone in Hamilton.
While the commission has found that incidents within the Hamilton control zone had fallen since airspace changes, not all user concerns have been resolved.
''Changes in the size and shape of the Hamilton control zone have shifted some visual flight rules with traffic congestion to prominent points outside the control zone, and likely increased the risk of collision in those areas.''
The Civil Aviation Authority — responsible for air safety — says it is satisfied that safety issues at Hamilton Aerodrome are ''being addressed''.
''The airspace around Hamilton can be very busy and we work closely with the aerodrome user group to manage air traffic congestion and make sure that competing demands for use of the airspace are managed safely.''
The airfield is one of the busiest in the country, with a high number of student pilots from flight training schools.
Academy director at L3 Commercial Training, Peter Stockdale, said because of the number of young trainee pilots, the air traffic control system needed to be well resourced.
''We had discussions for some time with Airways about the experience levels of air traffic controllers here. Since that incident they have done a bit to increase levels of experience in the tower,'' he said.
The Airways spokeswoman said Hamilton aerodrome had experienced significant growth which had been driven by the growth of the flight training school and scheduled traffic.
The aerodrome was now characterised by a large number of low-hour pilots with English as a second language and the challenge of integrating them with scheduled airline traffic.
''What this event shows is that when an aerodrome experiences significant growth, greater collaboration between the airport, aircraft operators, the regulator and air traffic control, coupled with regular reviews of existing processes, are required.''
She said changes that had been put in place since the event reflected a high degree of collaboration between groups at the airport.
The commission's report also found that the usual briefing procedure before conducting the assessment was not fully followed, and that this probably affected the team dynamics in the control tower.
The controller was ''not feeling the best'' on the day of the assessment and was nervous about it.
At a broader level, the commission found that Airways' tradition of posting recently qualified controllers with limited experience to Hamilton aerodrome had the potential to raise the risk profile of the air traffic control unit.
The commission was told repeatedly that the Hamilton unit was an undesirable place for controllers to work because of the heavy workload.
The commission identified the following safety issues:
• The standard of team resource management in the Hamilton air traffic control tower did not match good industry practice.
• Some aerodrome controllers are "over-controlling" visual flight rules traffic in and around the control zone, which is unnecessarily congesting the radio frequencies and risks causing difficulties for inexperienced pilots and those for whom English is their second language.
The Airways spokeswoman said that since 2015 there had been a number of changes.
''Airways has made a number of changes at Hamilton since December 2015. These include providing additional guidance around on-the-job training assessments, additional support structures including a mentoring system for staff at the tower and additional administrative resourcing.''
The key lessons arising from the inquiry were:
• Operational assessments in a team situation have the potential to alter the normal team dynamics. It is essential that assessments are properly managed and that every team member is clear on their responsibilities and their involvement in the assessment process.
• Clear, succinct and short radio communication between air traffic control and aircraft is pivotal to safe operations.
Hamilton Airport experienced a 13 per cent increase in passengers across core routes during 2017, with close to 335,000 air travellers going through the gate.
