New figures show NHS Grampian has sent more than 2,000 people to Glasgow, Edinburgh and Dundee for operations since 2012 to meet strict waiting time targets.
The embattled health board, which is grappling with an unprecedented recruitment crisis, said it hoped to cut spending on private healthcare with the opening of four new operating theatres at Aberdeen Royal Infirmary and Woodend Hospital.
But the spiralling bill has angered patient watchdogs, who say taxpayers' money should not benefit private healthcare companies.
Jean Turner, of Scotland Patients Association, said boards had to be open with the Scottish Government on the challenges they were facing in meeting waiting time targets. She said: "It always makes me angry when the NHS is spending taxpayers' money in the private sector like this.
"I feel it is bad management that we don't have the core staff to deal with this problem.
"The private healthcare sector is in it for the easy bits. They don't train doctors or nurses and any problems with treatment tend to be fixed by the NHS.
"It is unacceptable when the NHS's bill for private healthcare goes up like this.
"Health boards are obviously struggling to meet targets and they have to rely on the private sector to make it work."
New figures show that just under £4 million was spent in 2012-13 on private hospital bills, up more than tenfold on the year before, and last year the total came to £3.1 million.