How can Ryan Kankowski be off side?

The Brumbies throw into the line-out with Kankowski of the Sharks at the back of the line-out just inside the line that is 15 metres from the touch-line.

The Brumbies throw the ball into a line-out. Ben Hand of the Brumbies rises high and catches the ball. He then brings it down and then passes it back to Josh Valentine.

When the ball leaves Hand's hands the line-out is over. Then Kankowski is allowed to move forward towards the Brumbies.

Does Kankowski's cross the line-of-touch before the ball leaves Hands's hands?

It does not seem so.

Is there a maul in the line-out? No, because the Sharks do not join up with the Brumbies to form a line-out. No maul, there is no need for Kankowski to fall back to the hindmost feet on his side.

So how could he have been offside?

The only way he could have been offside was if he moved across the 15-metre line before the ball left Hands's hands, because that was leaving the line-out before it was over.

Look at the referee. He is watching the movement of Kankowski's feet. Look at Kankowski's dancing feet. Which way do they go?  They move infield while Hands still has the ball in his hands. Do they cross the line?

It would seem that his left foot is over the 15-metre line while his right foot is inside the line.

Is that enough to constitute offside and three points?

Law 19.8 (h) Where the line-out players must stand. The front of the line-out is not less than 5 metres from the touchline. The back of the line-out is not more than 15 metres from the touchline.
All line-out players must stand between these two points.
Sanction: Free Kick on the 15-metre line

The sanction here is a free kick because it is about forming the line-out - where players must be before the line-out starts. In the  Kankowski case, the line-out started when the ball left Stephen Moore's hands.

Law 19.14 OFFSIDE WHEN TAKING PART IN THE LINE-OUT
(e) No player of either team participating in the line-out may leave the line-out until it has ended.
Sanction: Penalty kick on the 15-metre line

Overstepping the 15-metre line before the line-out is over is leaving it - All line-out players must stand between these two points.

Is one foot in and one foot out enough to constitute leaving the line-out? That may well be the case. The law is not as clear on it as it is on touch, when one foot out is out, or at the goal-line when one foot in is in.

It is understandable that the referee penalised Kankowski.

But was it worth it?  Not really. Rugby is not an exact science.