In an attempt to maintain the elimination of COVID-19 in New Zealand, all international arrivals are required to spend 14 days in government-managed quarantine and to return a negative test result before being released . We model the testing, isolation and transmission of COVID-19 within quarantine facilities to estimate the risk of community outbreaks being seeded at the border . We use a simple branching process model for COVID-19 transmission that includes a time-dependent probability of a false-negative test result . We show that the combination of 14-day quarantine with two tests is highly effective in preventing an infectious case entering the community, provided there is no transmission within quarantine facilities . Shorter quarantine periods, or reliance on testing only with no quarantine, substantially increases the risk of an infectious case being released . We calculate the fraction of cases detected in the second week of their two-week stay and show that this may be a useful indicator of the likelihood of transmission occurring within quarantine facilities . Frontline staff working at the border risk exposure to infected individuals and this has the potential to lead to a community outbreak . We use the model to test surveillance strategies and evaluate the likely size of the outbreak at the time it is first detected . We conclude with some recommendations for managing the risk of potential future outbreaks originating from the border.