Difficult Xave, how long has it been in the pot? Could be pot-bound, which suggests it needs to go into a larger pot, or maybe you could give it a root-prune but being an Aussie native it'll not be happy about root disturbance. Treat it cautiously and you might be successful but re-pot it in a mix suitable for Aussie natives. And also give it a light (top) prune to compensate.

just a follow up from what Funnel web said
treat most native carefully when re-potting or rot-pruning.
I would also like to add if you do a light top prune or even if you don't, it good to give plant a TE feed (trace element) in pot.
Now yellowing on leave is a TE deficiency it is a magnesium deficiency, magnesium get leeched out of the soil in winter, and also can happen after flowering.

To fix the problem is to add one teaspoon of magnesium or ( Epsom salts), to a 9lt water can and give the plant a drink at soil level.

Now if you have yellowing on new and young leave, it a sign of Iron deficiency to fix this to need to put a teaspoon of Iron chelate in in the water can and water at soil leave

Please Note : the magnesium or Iron will not fix the leaves that are already yellow it will help the new growth leaves coming through.

If I was you I would go down to your local garden store or Nursery ask them about Re-potting banksia and what they think and also ask them. Do they have (JUST have a Complete Trace Elements feed)?

The Complete Trace Element feeds have all the TE in it including magnesium and Iron

Regards
Benjamin Manning
Canelands nursery

This post was edited by Benno88 on Wed, Oct 23, 13 at 8:15