iOS 9 was recently released and one of the lesser-mentioned features is Wi-Fi Assist. Wi-Fi Assist is turned on by default (probably without you knowing) and enables your phone or tablet to automatically use your cellular data if it thinks your Wi-Fi connection is performing poorly.
This is probably a really great feature in countries where unlimited cellular data is common, however unfortunately in Australia this probably does not include many of us.
If your Wi-Fi (or even the Wi-Fi at a public hotspot) doesn't perform very well you might be chewing up your data allowance without realising it.
My suggestion would be to monitor your cellular data usage very carefully. If you open your device Settings, then choose Mobile Data you can scroll down to see your data usage. If you scroll right to the bottom you can Reset Statistics to set the counter back to zero again. Keep an eye on it over the course of the month to make sure you're not going over your data limit.
If all that sounds like too much work and you don't want to risk using your valuable data, or receiving an excess usage fee then simply navigate to Settings, choose Mobile Data and scroll all the way to the bottom. Tap on Wi-Fi Assist to turn it off.
Turning the feature off completely might not be for everyone. This is a great feature, although your mileage may vary depending on your data allowance and willingness to monitor your usage.