The washout in Centurion means India and Australia will battle it out for one semi-final slot in Group A, with Pakistan already through. If Australia beat Pakistan on Wednesday, they'll obviously go through as toppers in their group, while Pakistan will take the second slot. It'll get more complicated, though, if Australia lose to Pakistan and India beat West Indies. Then, the margins of the wins will decide which team joins Pakistan in the last four.
The vital question is the margins of victories. Currently, the difference between the net run-rates of Australia and India appear vast: +1.00 for Australia, -1.08 for India. The good thing from India's point of view, though, is that those numbers have come about on the basis of one match only - the stats from no-result matches are excluded in the calculation of net run rates. Thus, Australia's positive net run-rate is because they've scored 50 more runs than they've conceded (in the win against West Indies), while India have conceded 54 more runs than they've scored (in the defeat against Pakistan).
For India to pip Australia, they need to ensure that they redress that balance; in other words, they need to make up for that combined difference of 104 runs. If, for example, Australia lose by 50 runs and India win by 54, the net run-rates of both teams will be exactly zero (in which case the tie-breaker will be balls per wicket for each team in completed matches). If the margin of either result is one run more, Australia's NRR will slip below India's.
That difference of 104 can be made up in any combination - if Australia lose by 20, India will need to win their game by 85 runs (assuming Australia chase and India bat first). If Australia lose by 104, a one-run win will suffice for India.
The logic is similar if Australia bat first and if India chase, though it'll be much tougher for India to overcome the NRR deficit: if Australia and West Indies both score 250 batting first, Pakistan and India will both need to chase the runs in about 40 overs for India's NRR to sneak ahead of Australia's.
It's a tall order for India, whose only advantage is that theirs is a day-night game on Wednesday, which means they'll know exactly what they need to do halfway into their match.