Tropical Cyclone Winston is expected to intensify to a category-four storm, the second highest level, when it strikes Tonga's northern islands later on Thursday, according to Tonga Meteorological Services.
The Tongan weather bureau said it expected winds to reach as high as 95-110km per hour, while heavy rains were also forecast.
Aid agencies said they were concerned cyclone Winston could cause severe damage after it hit northern islands earlier this week, especially after a prolonged El Nino-induced drought weakened livestock and crops across Tonga.
Heavy rains could also bring another severe impact even after the storm passed, aid agencies warned.
"Another deadly threat lies in the wake of Winston - the presence of Zika virus in Tonga could spread easily in a response situation," said Carlos Calderon, Pacific humanitarian manager for Oxfam New Zealand.
Oxfam said cyclone Winston could disrupt access to running drinking water and hygienic drainage systems or mosquito control measures such as nets, raising the risk of an increase in mosquito breeding and a fresh outbreak of the Zika virus.
Aid workers have begun removing mosquito larvae from water tanks, spraying affected areas to remove the threat of Zika, and distributing mosquito nets, with a focus on pregnant women.
Tonga declared a Zika outbreak earlier this month after five cases of the mosquito-borne illness were confirmed and another 259 suspected.
The World Health Organization declared the outbreak a global public health emergency on Feb 1, noting its association with two neurological disorders: microcephaly in babies and Guillain-Barre syndrome that can cause paralysis.
