1. Spark makes it clear: The Roaming Pack and Roaming Data Pack is activated after first roaming use in eligible destinations and will expire seven days after the next New Zealand midnight, after the first roaming use or full consumption of the pack inclusions
2. Vodafone makes it clear: If you purchased the daily Add-On before midnight NZ time, you will be able to use it until midnight NZ time the next day. If you purchased the weekly Add-On before midnight NZ time, you will be able to use it for the next 7 full days (NZ time).
3. 2degrees makes it clear: The applicable number of days that a 2degrees Roaming Pack is valid for is based on NZ standard time. For example, if you purchase a Globetrotter Pack at 9:00am on 1 January NZ standard time, the Pack is valid for 14 days until 11:59pm on 15 January standard NZ time.

Fixed-period plans start once purchased
If you want a 24-hour plan, 7-day plan or anything else, all networks activate your bundle as soon as you purchase it. This means if you buy a 7-day plan but take two days to get to your destination, you will only have five days of use left. Networks generally suggest you only purchase a plan when you land,

MoneyHub user Annabel describes her experience:
"My Spark week-long plan was due to run out on Monday, so I happily used the remaining 400 MB of data by browsing while I was on a train from Edinburgh to London. Little did I know that the plan ran out on Monday midnight New Zealand time, not UK time, so my 400 MB of browsing stung me with a $20 bill as I no longer has a 'pack' to use". 

Additional MB data use is costlier than a plan, but if you're connecting in a country not covered by a plan, you take a huge risk. Lost in Oslo and can't find WIFI? Loading a Google map may take 10 MB, which will cost $30 on Spark's casual rates. If you have apps updating in the background, you could find yourself using 20 MB - Spark will add $60 to your phone bill for the privilege. 

MoneyHub user Benjamin describes his experience:
"We were in Oslo about to leave for New Zealand. Our data plan has run out, and I didn't renew it as we were about to get on the plane. The hotel WIFI stopped working and I needed to buy an extra bag and print out the boarding passes to save money and time at the airport. I used my phone, thought nothing of it, and arrived back in Auckland with a $250 bill for excess data use for 70 MB of data. I would happily have paid $5 or $10 or even $15, but I thought $250 was a mistake - it wasn't".
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This means that if you don't have roaming activated, using the network you connect means you are 'legally agreeing to the charges' and will have to pay them when your bill comes. For example, if you land in Dubai, and get an SMS saying 'Welcome to Dubai, calls cost NZ$15/minute, data is $25/GB, SMS is $5 to send, free to receive...", this is exactly what you will pay. 

Same Countries, Different Roaming Rates
Between Spark and Vodafone, there are different roaming rates for the same country. For example, if you stop off in Dubai for five days, and use 1 GB of data, with a Spark roaming plan you will pay $20 or $15, depending on the plan you pick. If you are with Vodafone, which does not offer the UAE in its standard roaming plan, you would pay $250 (as it sells 200 MB of data for $50 as part of its 'Data Angel Overseas' package). But if you go prepay, Vodafone would charge an astonishing $10,000 for 1 GB.

Unlike Vodafone, Spark doesn't offer all-inclusive roaming packages for customers in Austria, Belgium, Greece, Netherlands, Norway, Portugal or Sweden. If you're in any of those countries, you will pay a whopping $3 per 1 MB if you are a Pay Monthly customer, or $5 per 1 MB if you are a Prepaid customer.

The lesson from this is to be 100% clear on what your roaming plan covers, and to make sure you get usage updates. Many providers offer a free TXT service to notify you when you've used 80% and 100% of your data. This means you will know exactly what you're spending and what you have left in your allowance. 

You may forget to hit airplane-model in the excitement of leaving New Zealand, but if you fly to Europe, or anywhere for that matter, you can incur some hefty bills. The same goes for cruises and sailboats. If you have data on, apps will connect and download, costing you money unless your plan covers the country the network is in. You can avoid this risk by setting your phone to airplane mode while flying, and being clear about what countries are covered in your plan (even if you don't plan to visit them).

Guidance from English regularity organisations suggests that mobile phones will connect to the strongest network available, and users should be careful using their phone close to a border or when flying. It states that a mobile device, unless set in 'manual network selection mode', is designed to connect a customer to the strongest mobile network available. If you are out at sea or in the air, and your phone connects to a network that is not covered by your plan, you will be charged. So even if your plan covers Montenegro but somehow you connect to an Albanian network, you will be billed for whatever data you use.

MoneyHub user Matt describes her experience:
"I flew from Singapore to London, left on data, and picked up 'Welcome to' messages from Malaysia, Oman, Azerbaijan, Turkey, Romania and Belgium. Half of these countries were not on my roaming plan, so I got a $100+ bill. It wasn't a great way to start my holiday."

Most networks have stopped offering in-flight roaming and instead direct you to the airline. It's a similar story for cruise ships as well. In any case, before connecting, make sure to check the costs if you’re taking a cruise or using In-flight roaming on the plane as they vary and will be different from your network's roaming rates.

The easiest way to have zero roaming charges is to turn off your phone's data connection. You can never be charged if you're not connected. Plenty of free WIFI exists - hotels, cafes, bars and even on some airlines (Air New Zealand is one of them). 
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Europe, Japan and America all have cheap and easy-to-activate prepaid SIM deals which are far more price competitive than New Zealand network roaming charges. In Europe, mobile network Lebara is everywhere and you can get a European-roaming plan for around £20 (~NZ$40) giving you 10GB. In Japan, 14-day unlimited data plans cost around US$80 (~NZ$120) which can be much cheaper than any NZ roaming deal. Best of all, you can continue to use your apps if you turn-off data on your current SIM, and use the local SIM for your data needs.