National's Nicola Willis says SkyCity "and the like" provided the party with data for its proposed tax on offshore gambling outlets.
If elected, National is hoping to create nearly $200 million in revenue through the gambling tax to help pay for its fiscal policy.
But critics have been nit-picking its way through the policy and claimed to have found a gap in the gambling tax, with party Revenue spokesperson Barbara Edmonds saying the proposal was already in place.
"National's less-than grand plan appears to have learned nothing from Bill English's tenure as Finance Minister when the tax on remote services was introduced," Edmonds said in a statement.
"Contrary to National's fiscal plan, there is no 'tax loophole' on online gambling from offshore. There is however a rather large loophole in the National Party plan and we challenge Nicola Willis to release the full costings and stop rolling the dice with her assumptions."
Speaking to AM on Thursday, Willis bit back at those claims - saying the current measures weren't well regulated.
"The reality is, the only people paying GST are those who've actually agreed to do that," she said.
"In practice, there's a lot of unlicensed, Wild West-type behaviour happening in the online gambling space.
"What we're proposing is a licensing regime so that you have to be licensed here and then we can chase you for your GST and your gambling duties."
As part of National's analysis for the tax, Willis said the party was provided with data by the likes of SkyCity regarding international, online operators.
Willis was confident the research would stack up.
"Those who've looked closely at this, you would imagine, would be New Zealand-based gambling operators including SkyCity and the like," she said.
"We know that they have estimated it and we know that they get market-based estimates of the size of that market. You can imagine, for people providing gambling here, they've looked very closely at the size of that market and they're very aware of it."
The gambling tax was one of four revenue measures announced on Thursday to help fund its $14.6 billion fiscal plan.
Other measures included partially lifting the ban on foreign property buyers but slapping them with a tax, scrapping the commercial building depreciation tax break and moving to an immigration model where the cost of processing visa applications was recovered through fees charged to applicants.