The hosts were almost gifted the opener inside 10 minutes when Dan Burn gave the ball away to McCormack, but Darren Randolph stayed big to deny the Leeds striker.
Jason Pearce had a good chance to break the deadlock shortly afterwards, but he could not keep his header down from close range.
McCormack was denied again by Randolph in the 13th minute before finally getting the better of the Birmingham keeper five minutes later.
Randolph came a long way to get to the ball but only cleared it as far as Austin, who passed it to McCormack to lift over the stranded keeper.
The same duo combined for the second goal, albeit in reversed roles. This time McCormack was the provider, swinging a cross into an unmarked Austin, who nodded in from six yards out.
McCormack was involved again in the third goal, which came right on the stroke of half time.
His initial shot was saved by Randolph, but the rebound fell straight to Smith, who wrong-footed the keeper to put his side 3-0 up at the break.