I have created a card game, the idea is somewhere between poker and slots. I hope to create a website to monetise the game but do not know whether I am able to patent the game in order to stop big companies from stealing the game. How effective is a patent at protecting a card game? Also, if so, then must I patent all small variants of the game also? It should be noted that I am from Australia where gambling websites are legal. Thanks.
The AusPat database is an excellent source for looking for Australian patents. I did a quick search for "card game" and came across 276 results fitting those keywords. Not all involve the classic generic-52-card-deck format, but some do.
A good example - and one that seems similar to your idea - is "Modified blackjack game using non-standard blackjack card values", by Davinder Signh Sandhu. There's no direct link to it from AusPat.1
So yes, you can patent a card game in Australia.
At the moment, I don't have enough information to answer your other two questions.
1 I found a similar American patent with more information, though I have no proof that the two are in similar formats.
While many card games are patented in the U.S., it has been almost impossible to patent a card game or a board game in the last few years. The movement against software patents has resulted in a very broad application "abstractness" being used as a rejection, even though the courts have not defined it.