At first she thought the body was a rubbish bag, she told the court today, before realising it was her colleague.
Merritt said there was a lot of blood on the ground and "a big halo" around Ross's head.
A blood-soaked hammer was allegedly found in a bin outside the Merritt family's Kaikorai home two days later by police. It was wrapped in a bloodied hoodie along with a glove, the court heard.
Those items were shown to Merritt in court today and she said she did not recognise them. However, she noticed the glove had an "A" on it.
After being spoken to by police on the day of Ross's death she said she returned home where she found the washing machine on and the defendant in the kitchen.
Merritt said her son was getting band aids out of the cupboard and she did not see his face until the next day.
She noticed scratches to his forehead and asked what had happened.
"He said he scratched himself in the night," Merritt said. "He had eczema, so he used to scratch himself a lot."
During the Crown's opening yesterday, prosecutor Richard Smith said the jury could infer Merritt's injuries came during a struggle with Ross.
DNA samples taken from under her fingernails turned up cells 430 times more likely to belong to the defendant than anyone else, he said.
Merritt also told the court she had noticed blood that had run down the bathroom cabinet, while having a bath the day after the alleged murder.
That blood was also tested after police searched the home on December 4 in 2015 and results showed it was highly likely it belonged to the victim.
Defence counsel Anne Stevens yesterday told the jury there were "surprising gaps" in the scientific evidence that she would explore during cross examination later in the trial.
Her client denied responsbility for Ross's death and the lawyer said anyone could have attacked her in the open area of the car park.
The trial continues.
