The defence department is obstructing an investigation into money spent by its former minister Lindiwe Sisulu for chartered flights on a Gulfstream jet, the DA said on Tuesday.

Defence Minister Nosiviwe Mapisa-Nqakula had to explain why for over five months the department had not handed over documents to the Auditor General, Democratic Alliance spokesman David Maynier said in a statement.

The department was not immediately available for comment.

In March, Maynier asked AG Kimi Makwetu to investigate the use of the Gulfstream jets by Sisulu, now human settlements minister.

This included how many flights and ferry flights she took, and whether each flight complied with the ministerial handbook.

"The department of defence appears to me to be obstructing the Auditor General's investigation into the scandal surrounding Lindiwe Sisulu's flights on the Gulfstream executive jets," he said.

Maynier would submit parliamentary questions to Mapisa-Nqakula to ask why documents had not been handed over, and who was obstructing the investigation. He would ask what action would be taken against the person found responsible.

On March 16, The Sunday Times reported that Sisulu had accused Mapisa-Nqakula of lying about the number of flights she took between September 2009 and February 2012.

According to letters and documents Sisulu racked up an R11 million bill for chartered flights on a Gulfstream jet while she was defence minister, the newspaper reported.

Mapisa-Nqakula, in a written response to Parliament in October 2012, said Sisulu took 203 flights on the private jet. Sisulu insisted she took only 35.

In an interview with the Sunday Times, Sisulu vowed to clear her name and confirmed she was considering legal action against her Cabinet colleague.

"I want that expunged from my records with the necessary apology and necessary remedy. I will not have it. I've worked very hard for this government and my reputation is not going to be sacrificed on something silly like that."