The pipe burst in the early hours causing the road to crack and lift on the A35 Lyndhurst Road at Ashurst between New Road and Whartons Lane.
Southern Water said supplies to 60 properties had been cut as engineers worked to find and fix the leak.
The firm said the hot weather can dry the soil out, causing pipes to shift and break.
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The road has partially re-opened with two-way temporary traffic lights in place while the work to repair the pipe is carried out.
Drivers have been warned to expect lengthy delays.
The nearby New Forest Inn was forced to close when water from the burst flooded into the pub.
A burst water pipe led to the closure of a section of one of the major routes through the New Forest.