Dave

This is an illegal maneuver and the referee should penalise the opposition for pushing before the ball is in.

Here is the IRB Laws definition:

(j) Stationary and parallel. Until the ball leaves the scrum half’s hands, the scrum must be stationary and the middle line must be parallel to the goal lines. A team must not shove the scrum away from the mark before the ball is thrown in.
Sanction: Free Kick

So, if this is happening your capitan can politely point this out to the ref and you could also brief your scrum to brace themselves to hold any intial drive or give ground to make it obvious to the ref that illegal play is occurring.

I hope this helps you.

Simon