Many of the great moral or social debates are around members' bills, such as prostitution law reform or Easter trading.
Opposition MPs and Government backbench MPs use the members' ballot in different ways. There tend to be four types of members' bills.
The first is the opposition MP who puts up a bill genuinely wanting to achieve a law change. Examples of these are the anti-smacking law and the proposed lobbyist registration law. Two bills proposed same sex marriage are in today's ballot, and fall into this category.
The second type is the bill put up by an opposition MP, mainly to politically embarrass or damage the Government. These tend to be reacting against something the Government is doing, and they want to force a vote in Parliament on it, so they can use it as a campaign weapon at the next election. One example was the bill requiring Parliament, not the Government, to determine if any land is removed from Schedule 4. They often do not necessarily even want the bill to pass - they want the Government to vote it down, so they can attack them for it.
The third type of bill is the Government MP who puts up a bill genuinely wanting to achieve a law change. One example just passed is the bill requiring charities to disclose what proportion of a donation goes to the charity and to the marketing contractor.
The fourth type of bill is the Government MP just doing a very minor uncontroversial law change on an issue which would never be a Cabinet priority. These are a form of blocking action. While it is desirable to get the law tidied up, they are submitted to the ballot, so there is less chance of an opposition MP's bill being selected.
Ministers can not submit bills to members' ballot, so even if every backbench MP submits a bill, opposition parties will normally have a greater chance of "winning" as all their MPs are eligible.
In pretty much every party, an MP needs the permission of their caucus to submit a members' bill into the ballot. So while they are proposed by an individual MP, they are agreed to by a caucus. This doesn't bind the caucus into voting for it, if it is a conscience issue, but it does mean the caucus agrees they are happy to have that debate.
The Government often dreads the members' ballot. Not because they are necessarily against the issue a bill deals with, but because the issue can dominate the political agenda. Take for example the issue of same sex marriage. There are two bills in the ballot to allow same sex couples to marry. That means there is around a one in seven or eight chance one of them will be drawn out.
Now quite a few National MPs (certainly not all though) support same sex marriage, and the Prime Minister himself has said he would vote for it - at least for first reading. But despite that, the PM will probably be hoping the bill does not get drawn out. Why?
The reason why is that if drawn, then for the next nine months the political agenda will be dominated by the same sex marriage issue. Lobby groups will form for and against. There will be marches and protests. There will be televised debates. The parliamentary debates will be keenly observed and scrutinised.
For the general public, they will think that the Government is focusing on the pros and cons of same sex marriage, instead of focusing on economic growth, paying off debt, improving schools and better hospitals. Few members of the public differentiate between what the Government is doing, and what Parliament is doing. For them, Parliament is the Government. So that is why Governments tend to prefer members' bills are on relatively non-controversial issues.
However it is because of that caution, that many of our great social reforms have happened because of the members' ballot. It is a good thing that Parliament can legislate its own issues regardless of the Government's priorities. We should thank our parliamentary forefathers who had the good sense to write the rules so that backbench MPs can promote legislation one day in six. Our country is the better for it.