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Bhatnagar came forward this morning after NZ Herald inquiries tied the donation to a house on Upland Rd in Auckland's Remuera.
The address was listed on the original donation declaration form which had been signed by Bridges, withdrawn and then submitted again without the Cathedral Club donation.
At the time the donation was made, the Upland Rd house was lived in and owned by Bhatnagar.
Bridges, who has denied any wrong-doing, said the donation and one other for $14,000 had been mistakenly attributed to his return when they were meant to be to the National Party as an organisation.
The name of the Cathedral Club emerged during the days-long purgative exit this week of Jami-Lee Ross from the National Party, during which he accused Bridges of corrupt handling of electoral donations.
Among the claims by Ross is that Bridges signed off an election return which included a $10,000 donation from the "Cathedral Club" even though he knew the identity of the donor.
Ross said: "The Electoral Act clearly states knowingly filing a false return is a corrupt practice. I know Simon filed a false return because Todd McLay and I spotted that false name in his return in January and suggested it needed to be tidied up.
"Simon Bridges knows exactly what Cathedral Club is. It was a name he used to hide a donation from a close friend of his. He claimed it was a clerical error. I call BS on that."
Bhatnagar and Bridges both appear in a photograph from a gathering the NZ Herald has confirmed was a meeting of the "Cathedral Club".
The 2006 gathering also includes National MP Simon O'Connor, Ross and polling company maestro David Farrar, whose work for the National Party is legendary.
Bhatnagar, who is known as a National Party supporter, said the group was a dinner club which started in 2001 and last met around 2008.
He said he was one of the organisers and had used the name to make the donation."These days, it's easier for me to donate to parties, causes and charities than it is to participate on committees and help in campaigns as I once did.
"I'd rather be following my passions of watching my children play football, collecting NZ art, and getting involved in some great NZ tech companies.
"So I'm happy to be noted as giving $10,000 to the National Party in 2017. In addition, I have supported candidates in National with lesser sized donations in the past."
Ross has not responded to calls for comment. Farrar said he had attended one meeting of the Cathedral Club in Auckland in 2006, at which the photograph featuring Bridges was taken.
It was reproduced on his website Kiwiblog at the time.
As he recalled, the group was a "social gathering for people who liked to talk about politics".
It was called the Cathedral Club because the room in the Auckland Club at which gatherings were held had pictures of churches and cathedrals on the walls.
Farrar said he could recall attending only once, to hear former Labour cabinet minister Michael Bassett, an author and historian, speak about the fourth Labour government.
"I don't recall paying to attend beyond the cost of dinner."
Other speakers included Winston Peters, former Labour Cabinet minister and Speaker Jonathan Hunt and then Act leader Rodney Hide.
The other donation - $14,000 - withdrawn from Bridges return was from Cubro Ltd. The managing director of Cubro is Logan Currie, who also runs an Exclusive Brethren school.
The Exclusive Brethren support of the National Party in the 2005 election - and claims of hidden donations - led to changes in electoral law.
The rules about declaring the identity of donors is different for parties and candidates. Candidate donations over $1500 must be identified, but the threshold is $15,000 for political parties.
University of Otago law professor Andrew Geddis said there was another level of disclosure which required the identities of donors to be revealed to the party if the value was more than $1500.
The requirement was then on the party to record the identity of the donor, although if less than $15,000, it did not need to make the identity public.
He said Bhatnagar's explanation that the name was used for his donation was puzzling, because the value of it meant his identity was always going to have to be revealed to the party but not to the public.
"From a strictly legal point of view, if the Act is being complied with, this makes no sense."
Ross also alleged the National Party leader committed electoral fraud over a different $100,000 donation; a claim Bridges denied as "baseless" with "zero chance of success given they are false".
Ross has said he will make a complaint to police today about the $100,000 donation, said to have been made by Chinese millionaire Yikun Zhang.
A spokesman for the National Party has confirmed Bridges was involved in the Cathedral Club and the donation came from Bhatnagar.
It does not explain why the name "Cathedral Club" was used.
"The National Party fully complies with the Electoral Act but has a long-standing policy of not discussing donations.
"Given the public interest in the Cathedral Club, however Aaron Bhatnagar has confirmed he is the donor.
"Simon and Aaron Bhatnagar are friends and Aaron is a well-known and strong supporter of the National Party.
"Simon was involved in the Cathedral Club which Aaron has explained was a dinner club for people with a common interest in politics.
"As the National Party has outlined, the donation was made to the National Party, not to Simon and the error was fixed as soon as it became known."