Arthur's Pass and Haast Pass both remained closed on Saturday night after continual heavy rain and rockfalls induring the last two days.
The Transport Agency's senior network manager, Mark Pinner, says State Highway 6, from Haast township to Makarora, opened at 10.30am but closed again at 2.30pm after heavy rain brought more rocks down onto the highway.
"At Arthur's Pass, our contractor has moved about 8000 tonnes of rock and material from a slip about 1km west of the Otira township, which closed State Highway 73 at midday yesterday.
"This is a massive slip. It is now estimated that up to 30,000 tonnes of rock has fallen onto the road at this site, with the slip being up to 6m high and material continuing to fall on the road as the slip is cleared."
He says it is now likely that SH73, from Arthur's Pass to Jacksons, will not re-open until Tuesday (27 May).
"We are looking at options to try and establish single-lane access through the slip area but a lot will depend on the heavy rain easing and how quickly we can clear the slip to make it safe."
Mr Pinner says at both sites the safety of the contractors and road users is the agency's top priority.
The next update on Haast Pass will be 10am on Sunday and 4pm on Sunday for Arthur's Pass.
A spokesperson for the agency said motorists travelling between the West Coast and Canterbury should travel via the Lewis Pass on State Highway 7, but warns high winds are also expected there.
The MetService has forecast bands of torrential rain passing over Fiordland and Southland until Sunday and people are warned to watch rising river levels.
Boil water notice
Residents in the township of Arthur's Pass are being advised to boil drinking water.
A Selwyn District Council spokesperson said the heavy rainfall has increased the turbidity in the source of the water supply, meaning the council cannot guarantee it is safe to drink.
While no E-coli contamination has been detected in the water, the treatment method may not be effective.