Mitchell Johnson has really put Australia in a prime position © Getty Images

The first Test at the WACA between Australia and South Africa has been one of changing fortunes, first the one side getting on top and then the other. But, there has been no bigger changer of fortune in this game than Mitchell Johnson.

The strong left-armer brought Australia right back into the game first of all and then put them into a position where they now have supremacy. They are looking like they are going to a take a really big first-innings lead, which will be a huge advantage in this match.

The South Africans took a long time to polish off the last wicket, and that was a bad sign. There wasn't much life about their bowling first thing in the morning and the Australian tailenders scored runs almost at will. So that was the first bad thing that happened to South Africa.

Then Neil McKenzie got frustrated with his own batting, he couldn't get things moving at all. His favourite shot is the pull shot, he played a horrible-flat footed attempt at a pull shot, skied it in the air and that was the first of the wickets for Mitchell Johnson. But much bigger things were in store for the left-armer later in the day.

Then it was Hashim Amla who changed the fortunes for South Africa. He came in full of intent and played very well. He played just the way you would expect a No. 3 to do; he took the control of the game. Graeme Smith is very restricted with his elbow problem; he is going really on fumes at the moment. He has a lot of guts and a lot of determination, but he is very restricted in his batting. But Amla took charge, he got the scoring-rate moving and this helped Smith along.

The partnership was looking really good until Jason Krejza produced a beauty - a well-flighted delivery drew Amla into the drive, spun through the gate and hit the off stump. It was a magnificent off-spin delivery. And then at the other end suddenly it was Mitchell Johnson again. Back into the attack, he got rid of Smith by a knock-on.

Two quick wickets and suddenly Australia were on the top in this fluctuating match. But then there was a very good, solid partnership - Jacques Kallis providing the solidarity and AB de Villiers the flair. This pair got things moving along very nicely for South Africa, both of them passed the 50-mark but once again, as the pattern has developed in this match: batsmen getting going, looking very good, looking like getting a century and then bang - suddenly they are out.

This was where Mitchell Johnson made the big change in the match. He came back on, got rid of AB de Villiers with a good slower delivery, an excellent catch by the wicketkeeper Brad Haddin diving forward. This gave Australia the much-needed break, because this partnership had put South Africa in a position where they must have been thinking, `We can get a good lead in then first innings'.

Then it was Kallis who went quickly after de Villiers. He too was caught behind by Haddin off Johnson. Johnson was on a roll, he took 5 for 2 as one batsman after another went down like skittles. And suddenly Mark Boucher at the other end was left with very little. In fact, Dale Steyn had to come in and face couple of balls, and there is only Makhaya Ntini left behind that. So Boucher has got to produce a miracle for South Africa.

Australia now, thanks to Mitchell Johnson, who has the best-ever figures for an Australian against South Africa, with some magnificent bowling - he got seven, his best in Test cricket - has really put Australia in a prime position. It now looks as though they are going to hold a lead of somewhere around 100 or maybe even a little more, and that will be a big advantage. Australia are holding the whip hand here at the WACA after day two.