"If you don't, they may cancel the policy, reduce the amount they'll pay you, or even reject your claim completely.
Hartley's claim was rejected by her insurer, as they said her illness was related to a pre-existing condition.
"Each insurer has different rules regarding what is considered a pre-existing condition. For some brands, you have to declare anything from the last two years, while with other brands it can be ten years or more," Ball said.
"If you've had surgery, or require ongoing treatment or medication, you often have to declare no matter how long ago the condition started. For peace of mind, we advise travellers to declare all their conditions."
However, some conditions like asthma, diabetes and hypertension can be automatically covered, provided you have been stable and haven't been admitted to hospital in the last year.
Travelinsurance.co.nz (TINZ) and 1Cover.co.nz have both recently launched online medical screening tools which assess a broader range of medical conditions. This means you can find out immediately if you can get coverage and if there is any increase in price.
If you're having trouble finding coverage for a pre-existing condition, Ball recommends shopping around.
"If you can't get coverage for your pre-existing condition through one insurer or it is prohibitively expensive, seek out a second opinion. Always read the Product Disclosure Statement, and if you're unsure about anything, call the insurer to check that the policy is right for you.
"We strongly recommend having travel insurance no matter where you're headed, and encourage travellers to be empowered advocates for their own health."