William Bourdon, who represents local human rights group Suaram over the French submarine deal, was deported from Malaysia on his last visit in 2011 when Najib's scandal-mired regime was still in power.
But after a historic change of government last year, he returned to the country to help a new investigation into the case.
Malaysia purchased two French-made Scorpene submarines for US$1.2 billion in 2002 while Najib was defence minister.
The French submarine maker Naval Group - then called DCNS - is alleged to have paid more than 114 million euros (US$128 million) in kickbacks to a shell company linked to Abdul Razak Baginda, a close Najib associate who brokered the deal.
Razak Baginda's Mongolian mistress Altantuya Shaariibuu, who was said to have demanded a cut for translating during negotiations, was shot dead and her body blown up with military-grade plastic explosives near Kuala Lumpur in 2006.
The case sank off the radar after a Malaysian court in 2008 cleared Razak Baginda of abetting the murder, sparking allegations of a huge cover-up to protect Najib.
Bourdon was set to meet with Malaysian Attorney-General Tommy Thomas on Tuesday to discuss the case, and anti-corruption officials.
"This is a complex case, you can imagine that they will like from us to update (them about) the proceedings in France," he told a press conference.
"Of course we will answer the questions as far as we can."
A French investigation was launched in 2010 after complaints from Suaram, and has already led to four French executives involved in the deal and Razak Baginda being charged. They all deny wrongdoing.
Najib was questioned by anti-graft officials over the case in November after Malaysia's new government took office and reopened investigations into the submarine deal.
The former premier, who is also embroiled in a separate scandal over the alleged looting of state fund 1MDB, denies any wrongdoing.
Najib's long-ruling coalition was voted out of power in May in large part due to the 1MDB scandal. The ex-leader has been arrested over the controversy and is due to stand trial.