Justice Minister Judith Collins is writing to the Independent Police Conduct Authority asking why it will not look into the police investigation of Malcolm Rewa, a serial rapist.
The 3rd Degree programme on TV3 reported on Wednesday night that shoddy police work allowed Rewa to get away with his crimes for years.
Ms Collins said she was surprised to hear the IPCA will not investigate.
She said the IPCA's decision might have something to do with an application by Teina Pora to appeal against his conviction for the murder of Susan Burdett.
Pora was found guilty in 1994 and again at a retrial in 2000 which was ordered after the semen in Ms Burdett's body was found to belong to Rewa.