'Descend to 5000' is the correct Australian phrase. Feet can be added to make certain it is an altitude being given.
The confusion comes because some people drop the 'to' thinking they are doing the right thing because 'there was that crash where they did/did not say to and the plane crashed and I think it was because of to and it was in (insert one of 10 countries grapevine style). '
Poms do it different as do the seppos.
Whether or not it is the best way to do it, the book (in Oz) says to say 'to' so everybody should. Simple.