Don't let the clear skies and sunshine fool you - it's Sydney's coldest morning in a year.

Just before sunrise, Sydney's Observatory Hill recorded a temperature of 5.5 degrees, which is 2.5 degrees below average, weatherzone.com.au senior forecaster Alex Zadnik said.

Mr Zadnik said it had not been so cold since June last year, when a minimum of 4.3 degrees was recorded.

Sydney's suburbs shivered through temperatures below zero just before 7am.

It was -2.1 degrees in Camden, -2 degrees in Campbelltown, -2 in Richmond, -1 in Bankstown and -1.8 in Holsworthy.