A man detained in China for almost two years is warning Canada to approach any possible trade deal between the two countries with "an abundance of caution."

After living in China for 30 years without incident, Kevin and Julia Garratt were arrested by Chinese security officials in 2014 and accused of spying and stealing military secrets, a charge Kevin Garratt said left the couple "dumbfounded."

Julia was released in February, 2015, but was put on bail with restrictions pending trial, and did not return to Canada until May 2016, while Kevin was not released until September, 2016.

Now living in Canada, the couple spoke about their experience Wednesday in an interview with CBC News Network's Power & Politics, as Prime Minister Justin Trudeau continues his tour of China to help lay the groundwork for a possible trade deal with Beijing.

"I would advise an abundance of caution, take our time in terms of free trade or whatever Canada wants to do with China," Kevin Garratt told guest host Catherine Cullen. "I think China is an amazing place, we loved it, and still do, but you need to take time to know what you are getting into."

Kevin and Julia Garrett recount their experience of detainment in China, and weigh in Canada's push for free trade talks with the world’s second largest economy 10:34

Julia Garratt said building a trusting relationship with China requires not just Canada to trust China, but for China to trust Canada. After three decades living and working in the country, she said she thought she had that trust — but it all came crashing down one day in August, 2014.

"I think it's a much more complex thing than should we trust China or not trust China? And I think that it just deserves some very deep consideration," she said.

Should Trudeau proceed with his efforts to forge a tighter bond with China, Julia Garratt said Canada should ensure it educates Canadians working in that country about what could happen to them if they run afoul of the law.

"If you do cross a boundary in China, or China decides to arrest you for some reason, what actually does it look like? What is the recourse and what could possibly happen?" she said. "We never realized that we could be held for two years in this capacity."

Arrested after dinner

The Garratts, who are from Vancouver and had lived in China since 1984, ran a popular coffee shop in the city of Dandong, near China's sensitive border with North Korea, and carried out Christian aid work in the area.

In 2014, the Garratts were out to dinner with another couple who were seeking the Canadians' advice about sending their child to Canada to study, when they were arrested leaving the restaurant.

"They kept telling us...: 'we're accusing you of being spies, we think you are spies' and... I was dumbfounded, how could anyone ever think that," Kevin said.

The Garratts' initial detention followed closely on accusations by the Canadian government that China had spied on Canadian federal agencies, including the National Research Council.

Kevin and Julia Dawn Garratt, shown here flanked by their son Peter and daughter Hannah, were detained in Aug. 2014 near the border with North Korea. They were accused of stealing Chinese military secrets. Julia Garratt was released on bail on Feb. 5, 2015. (Simeon Garratt)

The Garratts were held in a remote facility in separate rooms and did not see each other for the first few months, and after that only in "very, very carefully orchestrated" meetings. Julia said they did not meet with a lawyer until 340 days after they were arrested.

"We had very little understanding of the way the Chinese law plays out when there is some kind of an international incident like this," she said.

During a press conference with Trudeau in Bejiing on Aug. 31, 2016, Chinese Premier Li Keqiang was asked about Kevin Garratt's case. Li said Garratt would be treated humanely and his case would be handled in accordance with Chinese law.

Upon Garratt's release and deportation to Canada a few days later, Trudeau issued a statement welcoming the Chinese decision and said the Canadian government had been "seized" by the case "at the highest levels."

Influence could take time

When it comes to other Canadians being detained in China, Julia said Canada should deal with these cases at the same time as it negotiates a possible trade deal, being mindful that China adheres to Canadian principles and values.

"Obviously, you have to be working on every level to make sure that our Canadian values are compatible with the values that are being offered to us as part of the package of trade and ongoing development and relationships with China and Canada," she said.

Despite the challenges of dealing with China's legal system, Kevin said Canada should pursue closer ties with China, because "we just think it's better to engage with people than it is to cut them off."

Canada also has the potential, he said, of being able to influence China to improve its domestic record on human rights.

"I think there can be some influence, but I think you realize we are dealing with a very different system that has been around for a very long time, and that influence will be very slow, I would think, and it takes a lot of time, a lot of time."