Online users visiting the MetService website during this week's polar blast may have contracted a computer virus from malware hidden in online advertisements.

Weather experts are predicting a chilly weekend ahead as an outbreak of cold looks set to move up the country, bringing snow and gales.

Several roads have been reopened after a "15-to-20 year outbreak" of heavy snow and rain conditions sent a chill around New Zealand. The chilly weather is expected to move north over the country this evening.

Weather forecasters say worst of electrical storms over but heavy snow likely in south.

Thunderstorms are expected to batter the lower North Island and upper South Island, farmers near the Manawatu River are being warned to move stock because of flooding fears and survival kits might be needed in Southland.

Snow is causing havoc on the South Island's roads as wild weather continues to batter the country and while the storm is now peaking over the country, some places should brace for another rough 48 hours.

Thunderstorms are set to strike the southwest of the North Island this morning, with tornadoes possible, as one of the biggest storms of the year lashes the country.

The ash cloud from Chile's Mt Puyehue Cordon Caulle volcano has returned to cause more disruption to flights.

A bitterly cold southerly from the Antarctic will sweep up the country this week, bringing snow to all skifields.

A tornado, floods and king tides wreaked havoc across the country yesterday.