The NRL has issued premiers Manly with a "please explain" over their alcohol-fuelled season launch and has not ruled out taking action against the club.

The function held at the Manly Wharf Hotel last Friday was attended by sponsors, fans, players and officials and doubled as a celebration of the Sea Eagles' 2008 premiership and recent World Club Championship success.

It has been alleged that fullback Brett Stewart sexually assaulted a 17-year-old girl in the hours after the function, while backrower Anthony Watmough allegedly slapped a sponsor.

Stewart, who denies the allegations, is the subject of a police investigation.

Some players reportedly continued drinking at the premises after the official part of the function which had free beer.

Manly chief executive Grant Meyer admitted on Monday the drinking by some players was "unacceptable".

The club has now been ordered to hand a report to the NRL explaining why it should not be disciplined.

"Part of what the situation has thrown up has certainly raised some serious questions around the club function that was held at Manly," NRL chief executive David Gallop said.

"We are expecting a report from Manly, we are conducting our own investigation into that, but clubs need to know that where they are holding team functions and they are serving alcohol they need to manage that appropriately and if they don't manage it appropriately they can expect to get penalties from the league."

Asked if the NRL was considering fining Manly, Gallop replied: "Like I said, we're expecting a report from them.

"We're conducting our own inquiries but clearly there are some serious questions being thrown up about how alcohol was managed at that function."

Earlier on Monday, Mayer admitted it may not have been a good decision to hold the function at a public bar.

"Of course you second guess yourself a fair bit but I guess for the time I have been at the club it has been applauded by journalists from around the country in regards to our off field performance and attitude and culture and there was no reason to believe anything like this could happen," Mayer said.

"You look back and perhaps it wasn't a great decision and there is no doubt the drinking that took place on Friday was unacceptable."

Mayer said the club would take its own action over the "unacceptable" drinking later this week.

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