He said: 'We've done this trip once a year for the past six years.
"We've had reef sharks come up to us before, they might take a fish off your line after you shoot it, but nothing like this."
He explained that he was fishing in a channel off the northern-most tip of Queensland when he swam away from the main group.
He saw the shark swimming 50m below him, and tried to steer clear of it.
But he said the creature immediately turned to face him and started swimming towards him.
He was expecting the predator to give him a bump, as sharks often do when unsure about prey, so kept the spear gun ready.
It was only in the last 6m, Daniel said, that the shark charged straight at him.
With no time to think, he plunged the spear gun straight into its mouth before leaving the beast writhing underwater.
He added: "He ripped the gun out of my hand, ripped the line off too. I didn't realise the gun was stuck in his mouth so I was waiting for him to spit it out but then thought it wasn't worth it and swum away."
Daniel, who also manages a pesticide business, said he does not know what became of the shark.
But he has spoken to several shark experts and experienced spearfishers who say they have not seen that kind of aggressive behaviour before.