Harare, in arrears since 2001, paid back $120m (€100m) - more than a third of its outstanding debt - to the IMF in September after it threatened to expel the southern African country for non-payment.
It has since paid another $15m to the Washington-based lending body, and said it planned to clear the remaining $160m it owed by late next year.
Zimbabwe's Reserve Bank governor Gideon Gono has said the payback came from "free funds" and export earnings.
But, given the country's dire economic straits, the payment has prompted speculation and suspicion as to its source, with economists adding that Zimbabwe could not afford to spare hard currency given its current shortage.
Nowak said the crisis in Zimbabwe, which is grappling with record-high inflation, unemployment and severe food shortages on the backdrop of political tensions, was like a "cloud over the rest of the region".
But he added that foreign investors had realised that other countries in southern Africa were not planning to go the same route.