The firm had hired five new apprentices and five staff in preparation for the contract and would need at least 50 New Zealand-based staff to get the SkyCity work done - many of these based in Whangarei.
Mr Kirwan said the company had had its eye on the job for several years and had purchased "millions of dollars" of equipment in readiness.
"We went out and invested. We know there's no one else in the country now that has what we have."
Yesterday, the announcement that US firm Herrick - which also had bases in Thailand - would be involved with the build prompted criticism.
The Government had pointed to local job creation as a key benefit of the controversial SkyCity deal, in which the casino agreed to build the $402 million convention centre in exchange for a change to gambling law that allowed it to have more pokie machines.
E Tu union industry coordinator Joe Gallagher said it was "a disgrace" that this work would go overseas, while Labour Party leader Andrew Little said the Government should have included a clause in the deal to ensure work went to Kiwi companies and Kiwi workers
Mr Kirwan said in reality only a "small percentage" of the work would go offshore and the partnership allowed Culham to tap into expertise around large high-rise commercial buildings not currently available in New Zealand.
Culham was sending employees to the US to train with Herrick and Herrick workers would come to New Zealand.
"The idea is we'll send people over, suck up that information, and bring them back here," Mr Kirwan said.
Culham had a 58-year history of steel fabrication in Northland and had trained more than 700 apprentices in that time.
The company employed 230 people and was gearing up to hire more. Herrick and Culham have also just scored a second big job - 6500 tonnes of steel for a private project in downtown Auckland.
Culham's projects manager Hamish Wood, 33, had recently returned to Whangarei after studying in Canterbury and working overseas.
He spoke about the importance of having reputable firms based in Northland.
"I thought I'd come home [to Whangarei] and have to downgrade my career aspirations," he said.
"I never dreamed there would be a company of this calibre to work for."