So far, $91,236 has been raised in 23 hours by 1530 people.
The money will go towards support Day and Mulcahy's families in travelling to the UK, the open investigation into Day's death, and also the costs associated with bringing his body home to New Zealand.
Day's death has sent shockwaves through TVNZ, where he was a former camera operator and social media posts from his former colleagues show how valued he was.
A spokesperson for TVNZ said Day was a much loved member of the broadcaster's whānau.
"He was an immensely talented man, full of warmth and kindness. We are devastated by this news. Our thoughts are with his family during this terrible time. We will be respecting their right to privacy and will not be commenting further," the company said.
TVNZ Sunday correspondent Mark Crysell also wrote on Twitter that Day, alongside his dad Pete, was a valued team member.
"His beautiful eye and deep soul became pictures that lifted our stories to another level," said Crysell.
A close friend of Day's told the Herald yesterday that his death came as a huge shock.
Conor Connolly said he'd last caught up with Day in January for a coffee in Fulham, London.
"Joe was my best friend, we'd been best friends since intermediate school," he said.
"Disbelief and overwhelming sadness is how I'd describe my feelings."
