Rhinos chief executive Gary Hetherington has now released a statement, although he too refused to divulge any details on the nature of the incident.
"[Coach] Brian McDermott and myself have now had the opportunity to discuss the matter with the England management and with Zak himself and we now have a better understanding of the events leading up to Zak's departure from the England squad," read the statement.
"We believe Zak acted unprofessionally and in doing so, breached the Leeds Rhinos Player Code of Conduct. As a result, the club have issued him with a written warning and imposed a fine of £2,500."
"We are satisfied the matter has now been dealt with and Zak will report for training on November 25. We will continue to monitor Zak's future conduct and help him deal with his own issues; which are more to do with a lack of professionalism than anything else."
England take on Fiji in the final match of Group A on Saturday afternoon.