Parts of the South Island have been placed on alert as Tropical Cyclone Vaianu moves toward New Zealand, bringing the risk of heavy rain and hazardous conditions to the upper east of the island.

Tropical Cyclone Vaianu is expected to deliver the most severe impacts to the North Island, but forecasters have warned the upper South Island will not escape its reach.

MetService has upgraded a heavy rain watch for Eastern Marlborough, particularly between Blenheim and Kaikoura and around the Richmond Range, to an orange heavy rain warning. The warning is in place for 14 hours from 9am tomorrow.

The orange warning means streams and rivers may rise quickly, with a risk of surface flooding, slips, and dangerous driving conditions across affected areas.

New Zealand Transport Agency said it was preparing for possible disruption across the state highway network as the system approaches.

The agency said response crews would deal with slips, flooding, fallen trees and other hazards where it is safe to do so, and warned it would close roads if conditions put road users or workers at risk.

Officials are urging anyone planning to travel from tomorrow to monitor weather forecasts and road conditions closely, and to adjust plans if needed.

Transport authorities said updates on highway conditions would be provided as the situation develops.