Anticipation has been high for the moon's biggest and brightest display in nearly 70 years, with a similar showing not expected until November 25, 2034.

But BOM forecaster Sarah Chadwick said sky watchers in Sydney would likely be left hoping for an occasional gap in the clouds – at least until midnight.

"It looks like there's going to be a layer of cloud from a couple of thousand feet above the surface, to a bit higher than that, so it is going to be reasonably cloudy," Ms Chadwick said. "You might be lucky and get a couple of breaks but I'm thinking the odds are against you."

Other parts of NSW have a better outlook, with Weatherzone meteorologist Drew Casper-Richardson saying viewing conditions should be better in the north and far west.

This week's so-called supermoon is the second of three this year. A supermoon occurs when the moon is not only full, but orbiting close to earth.

On Monday night, the full moon will be the closest it has been to Earth since January 26, 1948. According to the Sydney Observatory, it will reach its closest point at 10.21pm AEDT, when it is just 356,509 km away.

However, in what may be some consolation to disappointed Sydneysiders, Perry Vlahos, the vice-president of the Astronomical Society of Victoria, said stargazers would probably not notice any difference between the three supermoons this year.

"The difference between other close moons would only be, in some instances, 100, 200 or 300 kilometres. To the human eye, that is almost imperceptible from a distance as great as that," he told Fairfax Media earlier this month.

The next supermoon is on Wednesday December 14. For those still willing to try for a glimpse of Monday night's moon, Mr Vlahos recommended any east-facing Sydney beach such as Manly or Bondi.

With Megan Levy