Do you know the joke to this punchline?

“It’s ok, that’s my D-a-a-a-a-a-a-a-a-d.” The word Dad is delivered the way a sheep says “B-a-a-a-a-a-a-a-a.” If I remember correctly this joke is pretty short and a little off color. I’ve been trying to remember this for months now. I hope you guys know. Thanks.

An Australian on holiday is taking a morning run through rural New Zealand. As he passes a farm, he spots something very odd. He runs to the farmhouse and pounds on the door.

A young boy answers the door. The Australian is taken aback at first: the boy has a beautiful, dense mop of curly, pure white hair. He recovers and says to the boy: “Oy! You might want to call the police, mate. There’s some weirdo shagging a sheep in your field.”

The boy responds: “It’s ok, that’s my D-a-a-a-a-a-a-a-a-d”

(ok, needs a bit of work on the delivery)

Shorter version I learned:

“Isn’t that your dad over there in the field, humping a sheep?”
“Naaa-aaa-aaaah.”