"He's tied her, bound her hands, and she managed to escape initially from that binding," said Acting Detective Senior Sergeant Andy King of Waitakere.
Mrs Corless was dragged to a room behind the till but managed to get free from the bindings while the attacker turned his attention to the till.
The attacker caught her, bound her again, and then beat her about the face a second time with fists.
He then took money from the till and left. He was in the RSA for less than 10 minutes.
Mr King said Mrs Corless was severely traumatised. "It was a very brutal and unprovoked attack. It was an aggravated robbery. We believe that was his motivation."
When the attacker left, Mrs Corless, who has worked at the RSA for 17 years, called police and a friend for help.
"When police arrived, she had blood on her face, was heavily bruised and in shock."
She was treated for her injuries in Waitakere Hospital.
Last night, she was recovering at home with her family.
Mr King described the assault as "pretty ugly" but ruled out any link with the triple RSA killings at Panmure 10 years ago.
"I worked on those homicides and my thoughts [today] are with the victims ... In my opinion there's no link at all to what happened 10 years ago."
He believed yesterday's robbery was an opportunist one.
Police were scrutinising CCTV footage from inside and outside the RSA but could not release a description of the man they were hunting beyond that he was wearing dark trousers and a hooded sweatshirt.
The national president of the RSA, Don McIver, said the organisation was devastated.
"We are sickened that such a brutal and cowardly act has happened and we are doing everything to support such a valued staff member of the Hobsonville RSA."
RSA members spoken to by the Herald said there had been several break-ins before. One member said a big television had been stolen.
A Hobsonville Rd resident said a client visiting her husband's business saw the attacker emerge from some bushes next to the RSA and jump a fence by the road.
Police dogs were later seen at a motel about 100m away.
* Anyone with information should call the Waitakere CIB on 09-839-0600 or alternatively call Crime Stoppers anonymously on 0800-555-111.