The New National Party has been asked by its former alliance partner to open its books to forensic auditors following claims that German fugitive Jurgen Harksen donated money to the Democratic Alliance.

In a letter to NNP deputy executive director Darryl Swanepoel on Thursday, the DA's finance director Jackie Hart said auditors Ernst and Young had been asked to do a forensic audit relating to donor income of the DA and DP's books in the Western Cape and nationally.

"As the NNP was part of the scene for some considerable time, I have been asked by our federal executive to enquire whether you would allow the auditor to perform the same task on the NNP books?," she wrote.

Addressing journalists in Cape Town, DA federal council chairman James Selfe said the request had been made because "there has been some occasions the accounts, although they were in the name of the DA, were in fact run as component accounts".

"I think if we are going to get to the bottom of this, then we are going to have to get to the bottom of the NNP accounts at that material time."

Swanepoel was not immediately available for comment.

However, Western Cape NNP spokesman Ludwig Andersen said the DA should sort out its own problems.

Harksen had clearly intended his donation for the DA.

Erik Marais, a suspended Absa official, had testified to the Desai Commission that the alleged Harksen donation was paid into the DA account and that he presented the deposit slips to the Scorpions as well as the remainder of the cash, Andersen said.

"Unlike (DA leader) Tony Leon, none of the national leaders of the NNP were ever invited to meet Mr Harksen, or attend any function with Mr Harksen, or to visit his home.

"None of the present leadership of the NNP in the Western Cape was invited to meet Mr Harksen."

Morkel was both the DA and the NNP's provincial leader at the time of the alleged donations, while the DA's chief fundraiser and Finance MEC at the time, Leon Markovitz, was also a former NNP member.

Andersen said the NNP's books were audited by PriceWaterhouseCoopers.

"If any question arises during any official investigation that is directed towards the NNP, which has not happened up to now, the NNP will gladly provide any answers to the relevant institution."

In a press conference earlier on Thursday, the DA announced five steps it said was aimed at getting to the truth of the matter.

"These allegations affect the credibility of the party, and they need to be dealt with head on," DA federal council chairman James Selfe said.

Among these steps was a forensic audit of the party's books.

Morkel and Markovitz had also requested that the same firm should audit their own personal accounts.

Selfe said he assumed this also entailed the Morkel Family Trust's books.

With acknowledgements to Angel Quintal and Sapa.