Can anyone tell me what this book is?

Ok I remember a much loved book I had as a kid, it was set on an australian ranch, had a couple of kids that rode horses, a greyhound called Monk (who liked to catch rabbits) and someone adult called Paul (I think) Paul was building a house and the 'twins' maybe helped by banging the nails in. That's all I can remember.

I have absolutely no idea who wrote it or what it was called and have been trying to track it down for years - can anyone help me??