The query can be simplified by eliminating two joins:
select .......
.......
from product p
join supplier s on p.suppliercode = s.suppliercode
JOIN SALES SA ON SA.ITEMCODE=P.ITEMCODE
where s.countrycode = 'NZ'
group by p.itemcode
limit 10
Afterwards, two dependent subqueries in the select clause can be reduced to one outer join:
select p.itemcode,
p.itemdescription,
round((SUM( CASE WHEN YEAR(s.date) = 2013
THEN SA.QUANTITY*P.SellingPrice
ELSE 0 END
)),2) As Sum2013,
round((SUM( CASE WHEN YEAR(s.date) = 2012
THEN SA.QUANTITY * P.SellingPrice
ELSE 0 END
)),2) As Sum2012
from product p
join supplier s on p.suppliercode = s.suppliercode
LEFT JOIN SALES SA ON SA.ITEMCODE=P.ITEMCODE
where s.countrycode = 'NZ'
group by p.itemcode
limit 10
Please try this query and let us know how it will perform.