The State Emergency Service is bracing for a busy 48 hours in the Hunter as a low pressure system moves south from the Mid North Coast, bringing heavy rain and possible thunderstorms.

The Weather Bureau says we can expect up to 80 millimetres today and 100 millimetres tomorrow, with isolated falls topping 200 millimetres.

A Severe Weather Warning has been issued for the region with intense rainfall which could lead to dangerous and life-threatening flash flooding and damaging winds with gusts reaching in excess of 90 kilometres per hour.

The region has already been lashed with rainfall, and the weather bureau says things will only get worse before conditions improve on Sunday.

Blacksmiths has recorded 187 millimetres since 9.00am on Thursday, Merewether 140 and Adamstown 120.

Several roads were flooded while buses are replacing trains between Maitland and Scone today due to flooding on the tracks.

The SES says moderate to major flooding is expected on the Patterson and Williams Rivers; minor to moderate flooding is likely at Wollombi Brook and on the Lower Hunter River while local and minor flooding is expected in the Newcastle and Lake Macquarie areas.

The severe weather has prompted the University of Newcastle to closed the Callaghan and Ourimbah campuses until Sunday night when the weather situation will be reviewed.