Melbourne captain Cameron Smith admitted being shocked but said he'd stand by Greg Inglis' decision to dump Brisbane and agree terms with South Sydney.

The Rabbitohs announced on Friday that Inglis had signed a three-year deal with the NRL club, six days after Brisbane withdrew their $1.2 million offer to the Queensland and Test superstar.

Inglis' former Melbourne teammate Smith said he had no idea why the 23-year-old changed his mind on a move to the six-time premiers.

Inglis had shaken hands with Broncos officials on a deal last Friday, but after meeting with Souths 24 hours later, he stunned the club by reneging on his verbal agreement.

"To be honest, I was a bit shocked he turned down the Broncos' offer to go to Sydney," said Smith.

"When he decided he was going to leave our club he said he wanted to come back to Brisbane to be with his fiancee.

"He seemed quite happy with his decision to go to the Broncos and we were all convinced that's where he was going.

"As a mate and a former teammate I'll stand behind his decision to go to Sydney where he obviously feels he's got better opportunities with the Rabbitohs."

Smith was unsure how much sway interim world boxing champion and former NRL player Anthony Mundine had in influencing Inglis' decision.

"Obviously he's going to take some advice from him because Mundine is a successful person and they've got a connection both being indigenous people," said Smith.

"I've also heard Russell Crowe is a pretty persuasive bloke and I'm pretty sure he would have been doing a bit of talking with Greg.

"Greg's only a young bloke and he can change his mind on things quite quickly.

"But he's also been around a while and he knows how to handle himself and he knows the consequences of making big decisions like that."

Smith said he hadn't been able to contact Inglis, who was in Melbourne to watch the Australian Masters.

"I haven't seen any interviews or much that has been in the papers, but a few of the boys said he looked a bit rattled with all the attention he's been getting the past week or so," said Smith.

"I'd just like to speak to him as a mate and see how he is going and how he's handling everything."

NRL chief executive David Gallop insists Souths still need to prove Inglis fits under the salary cap before the deal can be ratified.

Gallop said Souths had yet to lodge the contract with salary cap auditor Ian Schubert.

"I am glad GI appears to have resolved his future, his situation has been unusual and unfortunately the root of that goes back to the massive unravelling of the cap breach at the Storm," Gallop told AAP.

"This is compounded by a pretty loose agreement with Brisbane over the payment of his legal fees, and once a contract is lodged we will consider it against Souths' cap position.

"Clearly Souths will be aware that we'll have look at that and also what he has been paid previously.

"Obviously if he was being lodged as being on the books at $100,000 then alarm bells will start ringing, but all of that is a standard part of the salary cap process and one we use for every club."

Melbourne are still at loggerheads with Inglis over a legal bill for his successful defence of an assault charge from 2009.

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