They were in bed in a local motel when the quake hit.
"It was really scary. It just kept going ... it did not stop," Miss Brosnahan said.
She said they were thrown around the room, which was "trashed".
"Stuff was just going everywhere," she said. "Our room copped the brunt of it, the roof crashed and the outside wall was leaning.
"The bed was on the wrong side of the room, pretty much."
Initially, all she could think was "what is going on?".
"You realise in your head that it's an earthquake but everything you have learnt just goes out the window."
With the fear of a tsunami, the couple jumped in their rental car and headed for higher ground.
They spent the night in the car on the side of the road, not sleeping, as they listened to radio reports.
Yesterday, they remained stuck, with no way in or out due to severely damaged roads.
"There's been no communication about what's going on, not enough emergency services personnel," Miss Brosnahan said.
"I don't know how long we're going to be here for."
They had a quarter of a tank of gas left, with none of the town's three petrol stations open, and $40 cash.
The dairy opened briefly and they were able to grab water, muesli bars and chips, which they are rationing.
But they were fortunate - through a "friend of a friend of a friend" they had been offered a house to stay in.
"We're a lot better off than a lot of people here so [we're] pretty thankful."
Helicopters were starting to fly in, but she didn't know if they would be choppered out or have to wait for the roads to reopen.
She said while driving to find cellphone reception, they came across a campervan stuck in the road in a crack with "these poor older people who were freaking out".
She admitted to "freaking out" herself during the initial quake. And the constant aftershocks didn't help, she said.
But last night, she said all they could do was sit and wait, and look on the bright side - the sunset was beautiful and they knew they could have been a lot worse off.