I trust Mclaren to know Magnussen very very well, they are not often bringing a driver directly from a junior formula directly in to the team, they did with Kevin and dropped Perez in the process less than a year after hiring him. Australia is as far as I see it an outlier when being compared to Button, a street circuit which he have never driven and which Button have won on 3 times the last 5 years. I would expect that if anywhere, then here Button would have the upper leg.
Qualifying is not over, race have not been run so this may still be Kevin's weekend.
For the season I expect a pretty close run by the two drivers, there will be weekends where Button is the better, there will be weekends where Magnussen will be the stronger. Rightly or wrongly a generally held view of Button is that his comfort zone is not as wide as some of the other top drivers, that when the car is to his liking he is as strong as anyone out there, but when it is off he struggle where the more accomplished drivers can wring a result from the cars he is not able to. There is no real absolutes in these views and opinions, however personally I do see Button as a tad lower than Vettel, Hamilton and Alonso all of whom I see as able to bring more from the car, than what the car 'can really do' - this is obviously not correct if they get a result from it, then it is possible to get the result and that is what the car can do, is just harder and needs the talent they have.
The real test for Magnussen will be how well he adapt to F1 as a whole, how well he perform with the tools at hand, how well he compare to Button in setup, speed, qualifying and race. McLaren have gone all in on him, I expect that he will prove them right and by the end of the season will be seen at least as good as Botton. Pointswise between them it will to an extent depend on reliability, race-wise I think they will be almost even.