Reporting Ready Too Early
AIP says - Domestic aircraft should change to tower frequency
A) In the holding bay, or
B) Close to, or at, the holding point of the nominated runway, when ready for takeoff.
I suppose the definition of "close to" to the holding point isn't really nailed down and can be debated.
Having said that, if i'm moving an aircraft close to the holding point but holding short of a taxiway prior to the holding point, I usually add "remain on this frequency".
This shouldn't be necessary as if an aircraft hasn't been cleared to the holding point, they should stay on ground frequency as a matter of course. It is extra radio airtime wasted on a call that SHOULD be unnecessary, but unfortunately I've seen too many aircraft switch to tower frequency whilst holding short of a taxiway such that i feel it works better if i say it.
I understand the temptation to make an early ready call, as the order that aircraft call ready in does have some influence on the order in which they depart - it's not the be all and end all and many other factors are taken into account, but all other things being equal, it does play a role.
If aircraft would at least not switch frequencies whilst instructed to hold short of a taxiway, that would be a start. Not sure what the solution is beyond that.