"Premier Modise simply ignored the new policy and bought herself the most expensive car she could find," DA North West leader Chris Hattingh said.
He said Modise's excuse that the car had been ordered before Finance Minister Pravin Gordhan’s announcement was "nonsense".
"All it would have taken to cancel the order is one phone call to the dealer in Mahikeng. Instead, Premier Modise went through with it, even after the Treasury suggested a more affordable BMW 530 instead," he said.
Modise's office on Monday defended the acquisition.
Spokesperson Lesiba Kgwele said the vehicle was not acquired in defiance of a Cabinet directive.
He said media reports suggesting the purchase was a snub at Gordhan's call for cost-cutting were disingenuous.
Modise's office said the allegations were absurd and malicious because the vehicle was already on order when the finance minister announced austerity plans in his mid-term budget speech.
"The decision to purchase the BMW 750i that cost R1.3m was taken by the provincial department of public works, roads and transport and was below the 70% limit prescribed by the ministerial handbook," he said.
He added that the official vehicle used by Modise's predecessor had exceeded the maximum 120 000km it had to travel and experienced constant mechanical breakdowns that compromised the premier's security.
Hattingh called on Modise to appear before the North West public accounts committee to explain her decision to buy the car.
"This waste cannot continue while people live in poverty without proper service delivery," he said.