The chances of a wetter-than-average summer for eastern Australia are increasing, with the Bureau of Meteorology set to declare a La Nina weather pattern is likely to take hold in the Pacific.

The bureau's shift from neutral conditions to a "La Nina watch" will be formally stated in Tuesday's fortnightly update, Rob Webb, the head of the bureau's national forecast services, told Senate estimates on Monday.

Andrew Watkins, manager of climate prediction services at BOM, told Fairfax Media the declaration of a "watch" indicated at least a 50 per cent chance of a La Nina, although shy of the 70 per cent level that would prompt the declaration of an event as virtually certain.

"We're not quite up to the 70 per cent yet," Dr Watkins said.