A close friend of Mr Walker in Uganda, Ian Kimanje, said he was travelling to be with him "early in the morning to find out what is happening" and was in talks with a lawyer about the situation.
He had to establish where Mr Walker is being held and his lawyer was to petition police for his release.
"I have known Geoff for six years now and am like his best friend here. Any time he needs help he calls me. I always help. I just hope the police treat him right."
Mr Walker, 60, could not be reached by cellphone yesterday and had last made a post to Facebook early afternoon yesterday Ugandan time. The first social media report about his arrest was made later that night.
Mr Walker was on his latest humanitarian mission to the republic with the Side by Side project, working to help people in the village of Ludok, and Lions International.
He was the official photographer capturing images of a Wairarapa balloon ride in January 2012 that tragically hit powerlines and crashed in flames in a field in Carterton, killing all 11 people on board.
Mr Walker launched a legal battle without success over the rights to some of the images, which were widely published early last year. He had told media he travelled to Uganda to escape memories of the tragedy.
His Featherston niece Hope Sexton posted on Facebook late yesterday the family was grateful for the support at home and abroad. She said his family had canvassed "many agencies and are working tirelessly to try and get in contact with people over in Uganda".
"We are communicating with Ministry of Foreign Affairs and Trades to ascertain where he is, that he is safe and what the process is to ensure his safe release."
New Zealand First deputy Ron Mark told Radio New Zealand the Wairarapa family of Mr Walker, his friend, were "very worried and very concerned" and were working to establish the facts behind reports about his arrest.
An MFAT spokesman said yesterday the ministry "is aware of reports a New Zealander is being questioned by the police in Uganda" and staff were to contact Ugandan authorities.