Sabratha was a tourist town and not considered dangerous but it was within a 60km corridor between Tripoli and the Tunisian border notorious for criminal activity.
Prosecutors from Tripoli and Sabratha had been put on the case but had reported no progress.
The associate said the New Zealand and British Governments - as well as the victims' friends and families - needed to apply pressure on Libyan authorities so it didn't become a "cold case".
"You have to stay in touch with them and push them along a little bit and give them some motivation to continue the investigation."
Howie, a 46-year-old health worker from Wellington, was visiting Libya to see where De Salis, 48, had worked for the past six years, first at an oil and gas company and then an engineering firm.
The couple had met only six months before.
They were going to the United Kingdom before heading to New Zealand, where they had planned to live together.
"It seemed to be one of these whirlwind things," the source said.
"They got very serious, but it was cut short, unfortunately."
A spokesman for the Ministry of Foreign Affairs and Trade in Wellington said the New Zealand Government had asked Libya to fully investigate the deaths.
"Assurances were received from the Libyan authorities that this would occur."
The British Embassy in Tripoli was also in regular contact and offered assistance. "While we hope that those responsible will be brought to justice, we acknowledge the obvious challenges any police investigation in Libya faces, including a very difficult security environment in that part the country," the Foreign Affairs spokesman said.
Foreign Minister Murray McCully said he believed the appropriate overtures were being made.
"I'm confident that the Ministry and the New Zealand Embassy in Egypt have made clear to the Libyan authorities our expectation that the tragic deaths of Lynn Howie and Mark De Salis are fully investigated."
