The letter, released to Allied Press under the Official Information Act, says while he questions the "logic, the morality and the chances of a Tarras airport ever becoming a reality", the council can't "simply ignore the matter".
"CIAL has spent in the order of $45million buying the land and, therefore, we can only assume it is serious about the proposal to create an airport.
"For our part, QLDC intends to investigate what reasonable steps it [may] take to oppose CIAL's endeavours".
Boult's letter — also sent to his chief executive Mike Theelen, Queenstown Airport's chief executive Colin Keel and minority shareholder Auckland International Airport's Adrian Littlewood — is scathing of the plans by Christchurch Airport and says a wide-body capable international airport, a 90-minute drive from Queenstown "can only have its sights set to compete with QAC's Queenstown Airport and possibly Wanaka Airport in the future".
Boult, CIAL's chief executive from 2009 to 2013, said it was "quite surprising" to learn of Christchurch Airport's plans, about an hour before they were publicly announced in July.
"The greater surprise, though, is that a 75%-owned subsidiary of another council would choose to compete with another 75%-owned subsidiary of our council.
"I'm not sure that territorial authorities have that as one of their things to do; to compete with each other in a commercial sense."
"Effectively, that is what's happening here."
Boult wrote to Christchurch mayor Lianne Dalziel two or three weeks ago, "expressing my disappointment and asking for her views".
Dalziel responded yesterday.
Boult said he could not say what steps QLDC might take until he understood the views of Christchurch Airport's controlling shareholder.
A meeting between Boult and Christchurch Airport's chief executive, Malcolm Johns, was cancelled on Tuesday.
Christchurch Airport communications manager Yvonne Densem said it would be rescheduled.
