Uncle Tobys Vita Brits 1 kg box for $2.25 (less than 1/2 price, was $4.59) at Woolworths. Listed on page 12 of the Weekly Specials Catalogue.
They used to have this price every few months but haven't for a while unless I missed the last one. Coles also have the same price every few months, so they may have it at the same time otherwise you could ask them to price match it.
I much prefer Vita Brits to Weet-Bix (or the home brand equivalents which are 1.12 kg for $3.19 [$2.85 per kg). They're a more natural looking brown colour with a soft flaky texture compared to Weet-Bix which are bone coloured with a hard dry texture.
This price makes Vita Brits the cheapest breakfast cereal you can buy from Woolworths except for Home Brand Rolled/Quick Oats (Porridge) 750 g for $1.19 [$1.59 per kg). So if you want to save money, buy enough to last until winter and stack the boxes in the corner of your cupboard. The usual expiry dates IIRC are from 6 to 12 months ahead.
Far superior to Weetbix IMHO