Former defence minister Lindiwe Sisulu must produce proof that the tender process to buy a new presidential jet was sound, the Democratic Alliance said on Tuesday.
"Sisulu should produce documentary evidence that the Minister of Finance, Pravin Gordhan, approved the deviation from normal competitive bidding processes," DA MP David Maynier said.
Sisulu on Monday said she was "absolutely convinced" that the department had followed correct procurement processes regarding the R2-billion Boeing 777-200 Long Range business jet.
She said the defence department had taken advice from National Treasury as well as the accountant-general on the potential purchase, which had been making headlines since the weekend.
A spokesman for Sisulu, who is now minister of public service and administration, on Monday said she had called for the deal to be suspended to allow her successor at defence, Nosiviwe Mapisa-Nqakula, to redefine the department's priorities.
Maynier also took issue with this step, calling it an attempt to distance herself from the matter.
"This sounds too much like a useful pretext to extricate herself from a dubious deal to purchase a new jet for President (Jacob) Zuma," he said.
Die Burger newspaper reported that Sisulu had planned to use a refund the government had secured on the cancelled purchase of eight A400M Airbus military transport planes to buy the new jet.
The daily said Sisulu wrote to Gordhan last month, asking to be exempted from the normal tender rules because buying a new plane for the president was a "matter of national security".