Father-son rule
Main article: Father-Son Rule
To continue the traditions of association that a family has with a particular club, sons of former players are able to be selected by the same club as their father played with under the Father-Son Rule. The main eligibility requirements is that the sons father must have played 100 games for that club, although there are some variations in place for non-Victorian based clubs. Originally they were selected as pre-draft selections, then the rules were changed to force the clubs to use a third round draft selection to draft the son. Currently there is a bidding system, where any team that wishes to draft a son must allow all other clubs to bid for him, and then match the bid with their next available selection.
source: wiki
so from that i gather u just have to equal the highest bidder it seems