It also says people should shower before and after, and clean the surface used with alcoholic wipes.
The study also mentions that having sex with people who are together in quarantine is safer, but there is still a risk.
It says someone leaving the bubble to get groceries or run errands could come into contact with Covid sufferer and become an asymptomatic carrier, putting their partner at risk.
Researchers say the safest approach to sexual activity is to abstain, writing: "low risk for infection, though not feasible for many". Another option, they add, is masturbation.
New Zealand, however, only has one active Covid-19 case left, and there is no suggestion from local health authorities that couples should be wearing masks or abstaining from love-making.