"It was so loud, like a train about to go into the house or a giant earthquake rumble.
"Plus the airports anywhere within the next two hours drive are tiny."
O'Sullivan later found out, via her local Facebook group, that was actually a planned exercise by the Royal Australian Air Force.
The Australian military is in New Zealand this week conducting low-level and instrument flying. Despite the scheduled training, the plan still managed to catch locals by surprise.
A C-17A Globemaster III aircraft is conducting the low-level flying until this Thursday, April 18.
According to the Royal Australian Air Force, personnel and the aircraft from No. 36 Squadron at RAAF Base Amberley are currently based out of RNZAF Base Ohakea near Palmerston North, and flying over the east coast of the North Island and both coastlines of the South Island.
"The ability to fly over different terrains including valleys and mountains, is essential to RAAF C-17A operations and cannot be trained for in Australia," the air force says on its website.
"RAAF air mobility crews maintain close ties with the New Zealand Defence Force, evidenced by the recent co-operation in the Solomon Islands for that country's general election.
"Noise reduction and the environment are vital considerations in the planning and conduct of military activity, and air force appreciates the ongoing support of the New Zealand community and Defence Force."
O'Sullivan told the Herald that, despite the initial shock, it was a cool thing to witness.
"Given the peaceful environment we live in, it was quite a shock but incredibly awesome too," she said.
"Would have loved to have been on board. Must have been such an adrenaline rush for the pilot."