Batten down the hatches and garden trampolines, strong winds are forecast across Southland tomorrow.

MetService has issued a strong wind warning for much of the lower South Island, including Southland, Fiordland and Stewart Island, from 11am to 9pm, Thursday, 11 August.

That means strong wind gusts could damage trees, powerlines and unsecured structures. Driving may be hazardous, especially for high-sided vehicles and motorcycles.

Northwesterly gales are forecast to be severe in exposed places at times, with gusts reaching 120 km/h.

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There's also a heavy rain warning in place for Fiordland from noon until 9pm, with 70mm-90mm of rain expected to accumulate and peak rates of 10mm-20mm.

Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.

To prepare your property for high winds:

• Bring inside or tie down anything that can be broken or picked up by strong winds. If you have a trampoline, turn it upside down to minimise the surface area exposed to wind.

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• Remove any debris or loose items from around your property. Branches and firewood may become missiles in strong winds.

• Bring pets indoors. Many animals are unsettled by storms and it is more comforting and safer for them to be with you.

Keep up to date with the latest weather information at the MetService website.