Unpaid bills racked up by foreign patients at New Zealand's largest health board rose by a third to $18 million last year.
It left a $4.5 million black hole in the Auckland District Health Board's budget for the 2010/11 financial year.
Bad debt written off by the Counties Manukau District Health Board also jumped, to $5.4 million in the eight months to February 2012, up from $1.7 million the previous financial year.
Chief financial officer Brent Wiseman said increases were mainly the result of population growth and high numbers of overseas visitors passing through Auckland.
"[ADHB] has one of the highest amounts in the country of gross charges to ineligible patients due to its position as a key destination and transit point for visitors to New Zealand," he said.
Wiseman said bad debt did not affect patient care.
"If a patient is ineligible and acutely sick, they are treated in the same way as an eligible patient. Recovery of costs incurred happens later."
Counties health board spokeswoman Lauren Young agreed that rising population, combined with increasing medicine costs, were causing debts to skyrocket.
"If somebody turns up sick and in pain we are not going to ask for proof of citizenship."
She added that overseas debt was a "sensitive" issue: "I am loath to suggest people might jump on a plane and try and come and rort the system, but I guess that's possible."
Auckland spent half a million dollars trying to recover debts from overseas patients.
New Zealand has a reciprocal free health-care arrangement with residents from Australia and the UK, plus citizens of Tokelau, Niue and the Cook Islands. Most of those who leave hospital without paying are from other Pacific nations, South and Southeast Asia, Europe and South America.