As a first step to figuring out where all your data has gone, you can check your usage history at the account portal here: https://account.lightwire.co.nz/
Data can be consumed by programs running on your devices (Desktops / Laptops / Tablets / Smartphones / Media Centers) that you aren't even aware of. This can include automatic system updates and in some cases even malware (viruses etc).
Another common cause is young family members who might be downloading large files such as movies, games and music without telling their caregivers in advance. Always be wary during the school holidays if you have children in the house.
Also, with the advent of new video on demand services such as Netflix, it is common for data usage to be much higher than it once was. If you use any services such as this it may be something to factor in.
If you find that you are consistently going over your data limit, you can upgrade your plan in the account portal mentioned at the top of this article.
If you still need some clarity, you can call our friendly help desk staff who can check usage broken down into an hourly basis.