The AFL is investigating whether Essendon trialled players ahead of this week's rookie draft before lodging the appropriate paperwork, after warning clubs against conducting unapproved testing of draft prospects last month.

The Bombers advised the league that they had invited a small group of undrafted players to train with the club's VFL squad early this week, ahead of Wednesday's rookie draft, but it is understood the AFL is asking why the paperwork wasn't lodged until Tuesday morning.

It is standard for clubs to invite players to train after they miss out in the national draft before making a decision on who to target as rookies. The rookie draft was held less than a week after the national draft for the first time this year, and it is understood Essendon waited until it had gathered consent forms for all 12 players to train before lodging the full paperwork.

The league became aware before last month's national draft that some clubs - not Essendon - had conducted medical testing on players, which is against AFL rules.

The league subsequently warned clubs they were not permitted to ''request, invite, allow, require or direct'' a player who had attended a combine or state screening session to be tested or to do any form of training with either the club or an associated club. Clubs face a $10,000 fine for a first offence and $20,000 for subsequent offences. The Bombers did some athletic testing on a large group of players before the national draft, but none of those players had been invited to the national draft combine or a state-based testing session.

Clubs are allowed to conduct such testing on players not invited to attend a combine, provided the combines have been held, the players have finished school exams and that it has followed AFL guidelines or sought the league's approval.