Once the regular choice for couples, a double mattress, at 135cm wide and 188cm long, is now regarded as a good option for a single person who wants a little more space but, despite its name, is probably not large enough for two.
A couple sharing a double mattress would have significantly less room each than if they were sleeping in a single bed alone. Couples will usually opt for a queen or king size mattress.
Because of the standard length being the same as a single mattress, a double bed is also not regarded as suitable for tall people.
It's worth noting that sizes for double mattresses, queen size mattresses, king size mattresses and super king size mattresses vary between countries, so if you have imported your bed from overseas or brought it with you when you immigrated to New Zealand it pays to check specific measurements before buying a new mattress to fit.
Double mattresses are a good option for a sleepout, spare room or a student flat if room allows because the bedding is usually cheaper than for the larger sizes and the bed takes up less space in a multi-purpose room.