The Green Party laid the complaint with the police watchdog and confirmed this afternoon it had received a reply saying an investigation was imminent.
Police turned up at Hager's Wellington house in October last year with a search warrant after the journalist used information about blogger Cameron Slater, obtained from hacker Rawshark, in his book Dirty Politics.
Last week, the High Court ruled the search unlawful.
The Greens laid their complaint on Monday.
"We welcome the IPCA's prompt decision to investigate the decisions that led to the police warrant and unlawful search of Mr Hager's home," party co-leader Metiria Turei said.
"There are many unanswered questions from the Dirty Politics scandal, and why the police made the decision to search Mr Hager's home is one them.
"Given that the warrant and search on Mr Hager's house have been ruled unlawful, I asked the IPCA to investigate the decisions of senior-ranked police officials involved in applying for the warrant."
Ms Turei claimed that Hager's book uncovered a "dirty politics regime run out of the Prime Minister's office" and that John Key had not properly addressed those allegations.
"However, the Inspector-General of Intelligence [and Security] did investigate one of Hager's claims and confirmed the Prime Minister's staff had handed confidential information provided by the Security Intelligence Service to the attack blogger Cameron Slater."
