Fire and Rescue NSW Wollongong Inspector Chad Wallace said crews were using an industrial wet vacuum cleaner to mop up at the Victoria Square apartments, after damage to a sprinkler system in the unit.

Insp Wallace said three crews were sent to the apartment block around 10am, with all residents temporarily evacuated as a precaution.

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He said flooding had caused several fire alarms to be activated at properties across the CBD on Friday morning.

"Flooding through roof cavities has set off fire alarms, and our crews are now responding to multiple calls," he said.

Between 10 and 20 millimeters of rain is expected to fall over Wollongong on Friday.

Showers are expected to continue for the rest of the day and over the weekend.

Meantime NSW SES Wollongong team leader Menno Schaaf said crews had responded to eight calls since late yesterday in the region - mostly for water damage.

"Most of the damage has been caused by water leaking through roofs - a lot of people were unaware that there was damage due to the prolonged lack of rain, while others haven't been keeping their gutters clear," he said.

"That's the message we want to get across to people now with more heavy rain forecast for the next couple of days - make sure your roof is in good condition, clear your gutters and generally be prepared for heavy rainfall and storm activity."