Sydney will likely come close to notching its hottest day this late in the summer for 12 years - and there won't be much early relief.

Sydney awoke to a frigid Saturday morning, with temperatures dipping to a brisk 7.3 degrees in the CBD at 7:15am.

Australian customs officials destroyed two irreplaceable plant specimens that were being loaned to scientists by international institutions, prompting one of the institutions to suspend all transfers to Australian scientists.

Sydney could get its best weather for the year, with a week of gently warming days, lots of sunshine and little rain.

The mercury dropped to -5.1 degrees on Sunday morning, but it felt even colder, with an apparent temperative of -8.6 degrees just before the sun came up.