The IPC announced on Friday the Bloomfield Group's Rix's Creek mine had approval to continue operating until 2040 - hours before the public comment period was due to close.
The IPC then said "the purported determination was not valid" and extended the public comment deadline to midday on October 11.
"It just beggars belief that the IPC could issue an approval for a major project involving 300 jobs and then be forced to withdraw the approval just hours later due to an internal stuff-up," NSWMC CEO Stephen Galilee said.
"What should have been good news for NSW has turned diabolical, and once again the reputation of our state as an investment destination has been damaged."
Bloomfield had applied in May for a short-term consent extension for Rix's Creek, in case the approvals process for the overall continuation was not finalised by the NSW department of planning and environment before its current consent expired in mid-2019.
"Bloomfield acknowledges that the department's assessment process has been necessarily rigorous and exhaustive since the overall continuation consent application was lodged in 2015," the company said at the time.
IPC chair Professor Mary O'Kane said she had spoken to NSW planning minister Rob Stokes and welcomed a review into the IPC's operations, saying she was "conscious of the need for significant change" and said the IPC would work hard to "speed up" its determinations.
Bloomfield is an Australian-owned mining and engineering group and operates two openpit coal mines in the Hunter Valley, Rix's Creek northwest of Singleton and Bloomfield east of Maitland.