More than £50m was spent by the Department of Health sending patients outside Northern Ireland for treatment or paying for a specialist to travel to the province, over the last four years.
New figures show that between 2011 and 2015, a further £16m was spent on travel costs. It included both those on waiting lists and people needing medical procedures including assessments, surgery and other treatments.
However, the figures have been described as "nothing new" by doctors, who say for a population of around 1.8m, patients have always been sent to centres of excellence outside the province for the best treatment available.
But concern has been expressed over the impact that having to travel has on patients and families. It has led to calls to invest in the service to attract specialist staff so procedures and medical care can be carried out locally.
There are a number of reasons why patients would be transferred elsewhere for treatment, including cancer and transplant patients who are moved to specialist units. The figures were revealed in an answer to an Assembly question to the Health Minister by the UUP's Jo-Anne Dobson.
It showed the total costs - including treatment and travel - incurred relating to patients who actually travelled outside Northern Ireland from 2011/12-2014/15. Combined costs have gone from £16,774,783 four years ago, rising to £19,878,038 in 2014/15. Acute mental health care, which would include treatment for eating disorders, was not included.
Ms Dobson said she believed the situation is deteriorating across Northern Ireland.
"The fact there has been a 65% increase in patients being referred to other key specialists outside Northern Ireland, while the local NHS has lurched from one crisis to another, is indicative of a local service becoming increasingly unstable," she said.
"For many people, this travel will mean additional delays and greater stress and anxiety.
"I simply cannot understand why Simon Hamilton does not realise spending huge sums on specialist treatment outside Northern Ireland, often in some attempt to save money locally, is wholly counterproductive.
"In addition to paying £52m over the last four years for outside specialist treatment, the local Health Department has had to spend a further £16m on travel costs. I believe much of those additional costs would have been better spent on investing in our local hospitals and attracting and retaining the right staff so that these procedures and treatment could be carried out locally."
Ms Dobson said the response from the minister also "unsurprisingly" showed that while the number of specialist referrals has ballooned, the number of other routine patients being sent away to help ease pressures on the waiting lists has collapsed.
"However that has not been matched by an improvement of services locally, as the most recent publication of waiting times revealed that a record 373,000 people across Northern Ireland were waiting - the largest number in recent history."
But Dr George O'Neill, who is a former advisor to the Department of Health, said that sending patients elsewhere in the UK and to Dublin for treatment has been usual practice for years.
"There will always be a number of people who will be sent away, as the facilities aren't available here. It has always been the case.
"It works in the patients' favour to be honest, in the context of the money that we spend - even if you compare it to the prescription budget. It is just 10% of that."
A DHSSPS spokeswoman said "The Health and Social Care Trust in Northern Ireland refers patients to the independent sector for treatment to help to reduce patients' waiting times.
"Typically this happens where there is a gap between the demand for a service and the capacity in Northern Ireland's Health and Social Care Trusts.
"Tenders for waiting list initiatives are undertaken by Trusts."