"But we know this virus is tricky and has thwarted our systems before."
It was not yet known when exactly they caught the virus.
A group of 270 Russian and Ukrainian fishermen were expected to leave managed isolation at the Sudima Hotel at Christchurch Airport starting this week.
"The Community and Public Health team is working on an exit plan for the mariners, who will be transferred directly onto their ships once released from managed isolation," the ministry said in its update today.
Staff who worked at the Sudima Christchurch Airport Hotel since October 23 had been tested, and all 193 had returned negative results.
"All of case A's close contacts have returned a negative result, and all results received from Case B's close contacts are negative to date, with one result outstanding."
Cantabrians are being reminded about where they can get a test if needed.
Free testing continues to be available at general practices and urgent care facilities in Canterbury and at three Community-Based Assessment Centres (CBACs):
• Orchard Rd CBAC (near Airport), 174 Orchard Rd, (Off Harewood Rd). Open 9am-4pm, 7 days a week (walk-in/no referral required).
• Whānau Ora Community Clinic CBAC (Wainoni), 250 Pages Rd, Wainoni. Open 9am-4pm, 7 days a week (walk-in/no referral required).
• Ashburton Hospital site CBAC - 28 Elizabeth St, Ashburton hospital site. Open 10am-2pm, Tuesday, Thursday and Saturday only (walk-in/no referral required).
Community based testing clinical leader Dr Hannah Gordon said anyone who is concerned about the recently reported positive cases in Christchurch or thinks they may have been a contact of a case is welcome to attend.
"No appointment is necessary and testing is free for everyone. You do not need a referral to attend a testing centre and you can drive-up or walk-in," Gordon said.