Wet weather is hammering the top of the South Island as a severe-weather warning remains in place for Nelson.

MetService said heavy rain is expected to continue in the Nelson Ranges until 7am tomorrow thanks to an active front moving away over the upper North Island tonight and an associated deep low crossing central New Zealand tomorrow morning.

These weather systems are forecast to bring heavy rain to northern and western parts of both islands.

"In the last 12 hours Collingwood has had 67mm [of rain], most of that would have been in the early hours of the morning," said MetService meteorologist Brian Mercer.

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"More recently in Harwoods, that's had in the last hour 24mm, which is quite a significant fall."

The rain should clear by tomorrow morning, Mercer said.

Peak intensities of 20 to 30mm per hour were expected for a time overnight.

The southern fire service had received one case of a house flooding in the Tasman district this evening, which a spokesman said was not a major event.