Two critical by-elections are about to take place on opposite sides of the world. In a few weeks, in Auckland, the Roskill by-election will be held to elect a new MP to replace former long-serving Labour MP Phil Goff, following his election as Mayor of Auckland. And in west London, a by-election will be held in the seat of Richmond because of the resignation of the sitting Conservative MP Zac Goldsmith, who failed to get elected Mayor of London.

But the contrast does not stop there. In Roskill, the Labour Party is wooing voters by promising a more than one billion dollar infrastructure upgrade through a light rail system from Mount Roskill to the Auckland central city. However, in Richmond, Zac Goldsmith’s resignation was because of an infrastructure upgrade – the plan to build a third runway at Heathrow. Mr Goldsmith is hoping the Richmond by-election (which he is contesting as an independent) will become a negative referendum on the runway proposal. But, in Roskill, Labour’s Michael Wood is hoping the by-election will be a positive referendum on the light rail proposal. All of which goes to prove that in the topsy-turvy world of modern politics, by-elections can often produce the most perverse outcomes.

Because they are one-offs, with seldom any impact on the overall shape of government, by-elections often free up voters to indulge themselves in a way they would not normally do at a general election. By-elections are the opportunity to give the politicians the proverbial kick in the pants, without really upsetting the apple cart. While most by-elections see the incumbent party re-elected, there are sometimes occasions when real changes are made.

Seven times in the last 50 years by-elections in New Zealand have seen a seat move from the Government to the Opposition. (National lost Palmerston North to Labour in 1967; Marlborough to Labour in 1970; Rangitikei to Social Credit in Rangitikei; and, East Coast Bays lost to Social Credit in 1980. Labour lost Timaru to National in 1985; Te Tai Hauauru to the Maori Party in 2004; and, National lost Northland to New Zealand First in 2015.) No seat has ever gone the other way – from the Opposition to the Government.

But here is the rub. In only one of those instances did the governing party that lost the seat at a by-election also lose the next general election (National’s loss of Marlborough in 1970 heralded its landslide loss in 1972). In every other case, the governing party went on to serve another term in government. Also, with the obvious exclusion of Northland where it is too early to tell, the by-election change was not a temporary fling – it took at least two further general elections before the seat reverted to its previous holder.

A similar pattern exists in Britain, although from the time of the Orpington by-election in 1962 by-elections have often heralded a brief Liberal revival, or other third party successes, and, in the case of Scotland from the time of Winnie Ewing’s victory in the Hamilton by-election in 1967, the rise of the Scottish National Party.

All this adds to the pressure on the Labour Party in the Roskill by-election. Labour has not just to win, but has to win well. National, on the other hand, has no such expectations. In the extremely unlikely event it reverses every known trend and takes the seat, it will have triggered a political earthquake. Roskill is after all a fortress Labour seat, not held by National since 1957. Winning it would completely rewrite the history books. But it is most unlikely to happen, meaning National’s best hope is a close result which would be a pyrrhic victory for its candidate and a severe blow to Labour’s morale, just under a year out from the general election.

So pity the poor voters of Roskill who are likely to see more politicians over the next few weeks than most will see in a lifetime. The promises will flow – light rail here, probably upgraded schools and more housing there, none of which will ever come to pass. Thankfully, Roskill is a little further away from Auckland Airport than Richmond is from Heathrow, otherwise accelerating the runway extension would also be on the table. And then it would really be all on!