Marshall supports calls to give the NRL access to players' bank and phone records to ensure the governing body has greater control.
"As long as that (information) stayed private," he said.
Despite his strong stance against it, the former New Zealand Test captain believes problem gambling is limited to the "minority" of NRL players.
"Problem gambling is a problem in this country, first of all," Marshall said.
"I wouldn't say it's most players in the game. It becomes a problem when you're gambling money that you don't have or you're gambling money that you use to pay your bills or you're borrowing money to gamble.
"That's when it becomes a problem .... but the hardest thing is no players are going to say to you: 'I've got a gambling problem, can you help me?'
"That's hard to get out of players so I think (if) we can have more counselling or more one-on-one talks with people who may have a problem, it will benefit them."
Cronulla star Michael Ennis agrees "it's a fine line to invade on a teammate's privacy if you're unaware it's a problem".
"It's difficult at times to know whether it's just a social habit or whether there's actually a problem behind it," he said.
"Certainly if you know that it's a problem and there's been issues around it, then definitely it's up to not only the staff but also the senior guys to keep an eye on it."
- AAP