His tasks meant there was no opportunity for a pilot to have a rest or meal break.
"This is especially so given that a key feature of Jetstar operations is minimal turnaround time of aircraft and their crews while on the ground between revenue generating flights," the court decision said.
With the exception of quick toilet breaks when the aircraft's toilet was available, both pilots on the flight were required to be on the flight deck for the entire trip.
Meals were usually provided to flight crew once an aircraft was in its cruise phase although, particularly on some domestic sectors, this required rapid consumption of the meal because the cruise phase was short and the full attention of both pilots was required for the ascent and descent phases of flights.
Jetstar argued there were practical issues for not allowing time for breaks during the turnaround period.
The airline operated according to a low cost/low yield model which would be significantly undermined if Mr Greenslade and other Jetstar pilots were required to take rest and meal breaks during their work periods, the airline told the court.
If it was required to take breaks away from their duties and aircraft, that would adversely affect its service scheduling, causing additional cost to the airline or the loss of income to it as a result either of operating fewer flights or increasing aircraft and/or pilot numbers, Jetstar said.
However, the ERA rejected the airline's arguments and said its defences "fail comprehensively".
It requested the two parties come to a resolution within 60 days, and if there was an impasse the case would return to court.
The decision will filter across to all Jetstar pilots on a collective agreement.
Mr Greenslade, speaking from Australia today, preferred not to comment on the decision, only saying that Jetstar was a "great company to work for".
The New Zealand Air Line Pilots Association (NZALPA), however, said it was a "great result".
Mr Greenslade had the "absolute support" of all Jetstar pilots, said NZALPA general manager Virginia Mudie.
"We're very happy with the decision reached with Jetstar members," she said.
"I must stress that the relationship we have with the employers is very positive. But when you get into discussions like these there can be some unresolved issues.
"This makes things very clear for the company as well, so both parties should be pretty pleased."