Goulburn Ghost Tours has been taking groups to Belanglo, 120km (74 miles) south-west of Sydney.
NSW Premier Mike Baird said the tour was "utterly outrageous" while a victim support group has condemned it.
A company spokesman said the tours were run with "respect" and were not intended to upset anyone.
Milat is serving consecutive life sentences for murdering seven backpackers - two Britons, three Germans and two Australians - in the 1990s.
He had buried their bodies in Belanglo, which was also the site where a teenager related to Milat murdered a friend in 2010.
Goulburn Ghost Tours' website and Facebook page appeared to be offline on Tuesday.
A cached version of the site advertising the Extreme Terror Tour stated that visitors would visit Milat's "old stomping ground", calling it "this most terrifying of locations", and use "paranormal techniques" to solve a murder.
"Are you ready to turn grey overnight? Do you love to be frightened? Would you like to solve a crime? Then the GGT Extreme Terror Tour is for you!" it said.
Mr Baird was quoted by the Australian Broadcasting Corporation as saying the tour was "completely and utterly outrageous... it's not only in bad taste, it's just terrible. Horrendous."
Members of the NSW Victims of Crime Assistance League also said the tour was disrespectful of the victims and their families.
Louise Edwards of Goulburn Ghost Tours said the company runs the tours "with great respect".
She told the Sydney Morning Herald: "We are not there to promote a terrible tragedy.... We wanted to make sure it was sensitive. We really don't want to upset anyone."
It is unclear whether the tour will be allowed to continue.
ABC reported that the tours had been running since last June but the company had just applied for a permit to operate in the forest, claiming it had not realised it needed one previously.
But Mr Baird said he would reject any application by the company and warned that "if they operate illegally they will face the full force of the law".
An Australian firm has come under fire for conducting tours of a New South Wales forest where notorious serial killer Ivan Milat buried his victims.