The AFL have fined St Kilda $5,000 for having an extra player on the field during the warm-up of last Saturday night's semi-final against Collingwood at the MCG.

Saints midfielder Lenny Hayes had back spasms shortly before the game, prompting the club to send emergency Shane Birss out to warm up in case Hayes could not play.

Under AFL rules, a club cannot have an extra player on the field after final team sheets have been lodged, with a fine of up to $20,000 applicable.

AFL football operations manager Adrian Anderson said the league accepted the Saints had not deliberately tried to manipulate the rules.

But he said they had still clearly breached regulations and had not informed the AFL or the Magpies prior to Birss running out onto the ground.

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