"Mike hasn't lived here for so long - he said he came back for a good friend's funeral and spent a little bit of time here - and suddenly he's an expert on what the region needs.
"In my view that's uncharacteristic of Mike's very considered way of doing politics.
"Mike is unprecedented in his success. In my view he defined a president's role - he was there when Labour won three victories and is a very, very good operator and I have great respect for that."
ABHB chairwoman Rebecca Turner said Mr Nash "has the wrong end of the stick" in criticising the addition of Mr Williams.
Ms Turner said Mr Williams was not fronting ABHB's campaign to amalgamate local government, had a wide perspective and a "welcome enthusiasm for improving Hawke's Bay".
"We are not paying him - he wants to help us," she said.
"Stuart has the wrong end of the stick. It is so nice to have people like Mike who have a passion for Hawke's Bay and can see it needs to change."
Mr Williams said Lady Deborah Holmes gave Ms Turner his number "and she rang me up and asked if I was interested".
He said he grew up in Hawke's Bay, owned a house in Havelock North, planned to retire in the region and spent about one week a month in Hawke's Bay.
"I've been going back there for years," he said. "I've got my sister and brother there, my brother-in-law and sister-in-law there and I've owned property in the Bay for almost my entire life. How often does he want me to go there?"
Mr Williams said Hawke's Bay was falling behind other regions.
"I don't think amalgamation is the silver bullet but I do think it is a step in the right direction."
There were two campaigns to organise - getting submissions into the Local Government Commission and winning the referendum, he said.
"I can understand what Stuart is doing - he needs to get his name out there in advance of a general election and this is a good way to do it."