Waikouaiti farmer Jim Lawson flew to be with his sister, Vicky Rockefeller, in Melbourne on Tuesday.
• Ominous turn in missing millionaire case
"I'm just here for a bit of support. You need some family support at a time like this."
Mr Rockefeller (52), who is not related to the American Rockefeller dynasty, disappeared after leaving Melbourne Airport on the evening of January 21 after returning from a four-day business trip and police and his family are baffled as to his whereabouts.
His bank account and credit cards have not been touched and his car, a late model Toyota Prius, was found abandoned near the western Victorian town of Ballan.
She told ABC News her husband's disappearance was out of character and there was nothing to indicate where he was.
"We have no idea at all. He's level-headed, the business is going well and he's a real family man.
"You just can't imagine what it is like; it is just unbearable," she said of the three days spent wondering what had happened to her husband.
Mr Lawson said his brother-in-law's disappearance was unusual and unexpected and the family were still stumped about Mr Rockefeller's whereabouts, but they hoped the waiting would be over soon.
"It's a struggle though. It's a bloody difficult situation."
People had been wonderful to his sister and her teenage children, he said.
"There is food coming in every day. It's the children that we are really worried about."
He intended to stay as long as he was needed, but "there was not a lot you can do".
He expected the family would continue to need support after Mr Rockefeller was located.
"I think whatever happens, it's going to be awkward afterwards."
Their sister Julie, from Christchurch, was also in Melbourne to support Mrs Rockefeller, Mr Lawson said.
Because of the police investigation, he was unable to comment further, he said.
On Monday, police investigated an unconfirmed sighting of Mr Rockefeller in the nearby town of Gordon, where a dairy owner claimed a man resembling the millionaire had bought food.
There was another unconfirmed sighting of Mr Rockefeller yesterday walking on a main road in Kyneton, about one hour's drive north of Gordon, but police were unable to locate anyone fitting Mr Rockefeller's description.
Victorian police said in a statement yesterday they had received much information from the public, which they were sorting through.
News website www.theage.com.au reported yesterday Victorian police were confident, based on the evidence they had, Mr Rockefeller was still alive, but they still did not know where he was.
A suburban property investor in Australia, Mr Rockefeller was perhaps better known in New Zealand, where he was a director of casino group Sky City, chief financial officer at investment company Brierley Investments for many years and co-founder of Genesis Research and Development Corporation.
He met his future wife, Vicky Lawson, formerly of Waikouaiti, while they both worked for Brierleys in the 1980s.
The couple have two children and live in Melbourne.
Mrs Rockefeller still owns Kingscliff, the former home of Plunket founder Sir Truby King, in Karitane.
Mr Rockefeller was last in Karitane with his wife for Christmas and New Year 2008-09.
