Mr Williamson was invited on to the Ellen show after his speech in Parliament on the same-sex marriage bill became a social media sensation, but he believed having the trip paid for by producers was against ministerial rules.
However, John Key said yesterday that he had discussed the payment issue with Mr Williamson and given him the green light to appear.
"I'm required to give him approval under the ministerial and cabinet guidelines but I've done that, so if he wants to attend then he would be free to attend."
Mr Key said he understood the show paid guests' airfares and possibly an additional amount for appearing.
"I've made it clear to him that if that is the case he'd need to donate that [appearance payment] to charity, but outside of that, yes, he'd be free to go."
Clips of Mr Williamson's speech have had 1.5 million views on YouTube, and there were now versions with Spanish and Chinese subtitles. It was tweeted about by celebrities including DeGeneres, Stephen Fry, Perez Hilton and Ronan Keating.
A spokesman for Mr Williamson said they were waiting to hear back from the show's producers for more details.
In his speech on Wednesday, Mr Williamson, the long-standing MP for Pakuranga, made humorous references to "a big gay rainbow" over his electorate and said the Marriage Amendment Bill was a positive step.
He has since been getting accustomed to his newfound status as a poster boy for gay rights, for which he has received praise from the United Kingdom, Australia and America, offers to stand in as Governor in several states as well as appearances on various television shows.
The already married Mr Williamson said the New York Times called him one of the few "openly gay" MPs in New Zealand.
"It's gone a bit far," he said. "My wife wanted to know whether the New York Times knew something more than I did."
Green MP Kevin Hague, who helped Labour's Louisa Wall with the bill, said there were no sour grapes that Mr Williamson was getting all the attention.
"Louisa and I - and this is tongue in cheek - gave pretty good speeches too but at every stage we've been upstaged by straight National Party men. There was Paul Hutchison in the first reading, Chris Auchinvole in the second reading and now Maurice Williamson. But there's no resentment about that. It's funny, that's all."
He said some people might have been surprised by Mr Williamson, but in Mr Hague's time heading the Aids Foundation in the 1990s he had worked with Mr Williamson as Associate Health Minister.
"He has always been progressive on issues like gay rights, including supporting needle and syringe exchange when it was not popular."
- Claire Trevett and APNZ
