Can anyone explain the ending?

The man in the hockey mask is actually Robbo, the policeman played by Robert Mammone.
Most of your other questions and problems are answered- or not answered, in the case of the dogs- in other threads here,

Major, major spoilers!! Do not read until you have seen the movie.

I too enjoyed the movie, watched it in one sitting, didn't get bored or yawn once, understood every word as I watched it with sub-titles due to a crappy (or crapped out) sound card in my aging laptop. I too was left wondering.

I didn't recognize the hockey-mask fellow. I thought it was going to be the Police Sergeant, then when it was not him, thought it must have been the farmer who owned the land 'as far as you could see', the father of the 'roo shooter' and owner of the utility truck.

To me, the movie was a cross between Chuck Norris' 'One Wolf McQuade' - a deep-thinking loner, new in town, comes to solve the mystery - and a Jason Statham or Liam Neeson over-the-top, Gunfight-at-the-OK-Corral where our hero is a crack shot able to kill countless bad guys who are not.

From the start, the movie required a suspension of disbelief. A truck driver stops at a bridge relatively close to town to check his tires - why not drive a little further to a well-lit area? There, he is able to smell a corpse in a culvert a hundred feet away when the local (enhanced?) wild dogs could not.

At the shootout on Slaughter Hill:

I thought that Johnno was crooked as, when they were in the Chinese restaurant he tells Jay that he wishes he could find what was missing, and warns him to look out for his daughter. When Jay finds what's missing, the house ransacked and vacant, he makes a phone call, I assumed to Johnno to tell him he found the dope.

I wrongly assumed that they would exchange the dope for the daughter.

It appears Johnno comes to the rescue as a marksman but assumes a position shown twice previously to be incorrect, unable to rest his rifle on something solid. Not unusual for this Statham/Neeson genre, the good guy wipes out all the bad guys, such poor shots that Jay can drive off without so much as a hole in his radiator.

Off-base again, I assume, as I thought that when Jay drove to his ex-wife's neighborhood to find the mother and daughter on the side of the road, we had a Hollywood ending where the two errant family members had seen the light and wanted to re-join our hero and his ways. Fade to black.

The common thought in my post? "Assume". As the OP stated, there was no 'resolution of the plot'. Just the same, I've yawned my way through a lot worse.

Thanks for this. So why does his ex-wife's house get ransacked too? We see her sitting outside it, despondent and with a beer bottle after Jay explores it. I naturally assumed that this was where he later found the drugs (rather than the coincidence of two houses getting ransacked!)

I saw this movie today, at the cinema, and I was confused about some of the same things. I missed some of the dialogue, especially at the beginning, because of the Australian accents. Wikipedia has a detailed plot synopsis that explains a few things: http://en.wikipedia.org/w/index.php?title=Mystery_Road&oldid=63482 4347
I guess that Johnno (Hugo Weaving) and Robbo (the cop revealed to have been the hockey mask guy) had a falling out, with Johnno deciding to do the right thing in the end. But the earlier scene, on the street, was Johnno essentially confessing that he was the one who killed Julie Mason, albeit accidentally?

I doubt even the screenwriter knows what was going on.

Wasn't the guy with the hockey mask the cop that got out of the cop car when Johnno first speaks to Jay when he pulled him over....?

Swing away, Merrill....Merrill, swing away...

Here's my take on it - CONTAINS SPOILERS!

Like a lot of noir films the plot contains a lot of twists and often requires a bit of thought at the end to finally connect all the pieces of the jigsaw or even a second viewing.

I see a corrupt police drug squad which the officer who had died prior to the start of the film had suspicions about and the man in charge (the Sarge) content to look the other way.
The guys death was referenced at the retiring cop's leaving do, in hindsight by Johno when he asked Swan if he'd ever killed someone and later on the dead cops wife said it was another cop who had called him on the night of his death.

Then you have the drugs theft and double theft by first Wayne and then the three girls with Swan eventually finding the "drugs/McGuffin" at Julie's house, thus setting up a final showdown and taking the gamble that Johno wants to right the wrong of the year before - which in noir tradition will require his death.

I suppose you have to ascribe Crystal's survival to Swan wearing the white hat - although I don't forsee a family reunion...........

There were still a few unasnswered questions (to me anyway)
1 Wayne - presumably killed offscreen
2 The wild dog motiff was never really answered
3 Could Swann really just drive off with presumably half a dozen bodies to account for?

Having said that I really enjoyed the film and wouldn't mind seeing the charachter reprised........

The ending was the only thing killing me. After watching it and going back and forth. Where the ex wife and child at wasn't their house.

So they must had been waiting for him , in front of his house. That's why the puppy faces.