The train yards are below the building level and sound is minimal: a hovering (or orbiting) helicopter will put blade noise down into the stadium through the open roof and such noise will be amplified significantly.
It was a red helicopter, colour scheme of a local operator but not necessarily from that company. Not too long ago (OK, 15 years?!) there was a restricted area over the Melbourne Tennis Centre for the duration of the Open: that was lifted on the understanding that local operators used some CDF and kept clear. It would seem that someone has lost a bit of corporate knowledge and put us all in the doghouse: expect a reintroduction of the restricted area?
And for the tennis snowflakes to complain they can't hear the ball hit the racquet while they allow grunting by their own players specifically done to mask the same sound smacks of hypocrisy of the highest order!