What can be done if two out of three board members collude to increase their salaries without informing the other director or other shareholders?

You can try to file a shareholder derivative lawsuit http://www.deacons.com.hk/eng/knowledge/knowledge_470.htm#4 The other thing is that HK company law and UK company law are increasingly divergent.

If corporate bylaws require their raises to be approved by the board, voice your objection at the next board meeting. Insist on a claw back from the time they started receiving the raise until it is properly approved. If they refuse, organize the minority shockholders to sue.

Agreeing with Aakanksha, but breach of fiduciary duty seems to be a pretty obvious cause of action.  Officers have a fiduciary duty to the company, which means they must act in the best interests of the company, and cannot engage in self dealing.  If the officers colluded to increase their salaries, that is self dealing.  Whether they breached their fiduciary duty depends on the facts.

You have not given a lot of details so I would inform you on the face value of what you say. You say that those two directors colluded among themselves, while this prima facie doesn't seem illegal because directors have the power to resolve whatever they want with a extra ordinary majority of two third. Which is fulfilled in this case. So in order proceed against them according to company law there has to be a rule which specifically debars such kind of resolution. But if there isn't anything in the law, if which I am pretty sure that there isn't, you can proceed against them for BREACH OF TRUST. Since you say that the law is similar to that of england there is a possibility that you get some relief. Breach of Trust usually occurs when someone who is under trust for something or is suppose to follow certain principles of equity works against it. P.S.- This answer is for information purposes only and should not be considered a legal advice to be acted upon. Consult an attorney before starting to proceed.