SPCA chief executive officer Ric Odom said Manawatu was a "likely option" but it was premature to say what would happen.
Mr Odom said the SPCA Wairarapa committee decided it was best to become part of a regional structure and the "SPCA Manawatu is one likely option".
"The national board will consider the request and, assuming they approve it, they will determine how the administration is best applied. In this case it is likely that the board will ask Manawatu to exercise governance and management."
Mr Odom said the letter request from the Wairarapa SPCA would be sent on to the national board and "probably wouldn't take long to resolve".
He said Mr Sewell had "got ahead of himself" and it was "not a done deal".
SPCA Wairarapa had been doing a "pretty hard job for a long time and they have been through a lot".
Mr Odom said the SPCA Wairarapa committee had made a courageous decision in the best interest of the animals. "Sometimes, in smaller centres it can be hard to cover all the things that need to be done."
By linking with a big centre there would be benefits. Having more resources would mean inspector services, knowledge and expertise around animal handling, policy making, health and safety and fundraising concerns could be dealt with more efficiently.
Mr Odom said a new manager and senior inspector for SPCA Wairarapa had been appointed. The manager will be Ben Lakomy from the SPCA Wellington.
"He's a really experienced animal welfare inspector." Mr Odom wants to assure the public that the SPCA Wairarapa will be business as usual.
"They have a got a very dedicated group of volunteers, so the animals will still be looked after.
"I don't think people of Wairarapa will notice a difference, but maybe an improvement as they are part of a bigger structure."
Rob Sewell could not be reached for comment.