An internal Samsung email obtained by the Herald on Sunday shows Williamson inquired about the company's programme of "gifting" products to politicians.
The email shows Williamson wanted the phone "urgently" ahead of an overseas trip in May last year. "He is travelling on Sat, and wants his phone in a hurry," it said.
The Registrar of Pecuniary and Other Specified Interests, Sir Maarten Wevers, said it should be on the register if Williamson had more than $500 worth of benefit from the phone over a 12-month period.
"If you were lending someone a house for a year or a car for a year and it has a value of more than $500, I would expect that to be included in your return," he said. "If you're in doubt you should declare. That's the rule of thumb."
Sir Maarten said he would check with the Office of the Clerk on Monday to see whether there were for any precedents for this sort of thing
Constitutional law expert Andrew Geddis said Williamson was wrong to say it was not a gift. "There's still a gift involved in that he's gaining free the use of this device for the period in which it is in his possession," the Otago University professor said.
Williamson resigned his ministerial portfolios last week after being accused of interfering in a police prosecution against National Party donor Donghua Liu.
Labour Party leader David Cunliffe last night said guidelines around declaration of gifts by ministers were to ensure public confidence in the system.
Adams said the phone she received was a gift and she did not plan to return it.
The Prime Minister and Samsung declined to comment.