The mortgage amount is $$745906$

$1.$Initial fortnightly payment of a $25-$year mortgage using the PVA formula.

$2.$ Fortnightly payment in Year $6-10$ if the interest rate is increased by $1\mathrm{.}5\%$ after $5$ years using the PVA formula.

$3.$ Fortnightly payment from Year $11-15$ if the interest rate is further increased by $0\mathrm{.}6\%$ after $10$ years using the PVA formula.

$4.$ Fortnightly payment from Year $1625$ if the interest rate is further increased by $1\mathrm{.}2\%$ after $15$ years using the PVA formula.

$5.$ Effective annual interest rate for each period: Year $1-5,$ Year $6-10,$ Year $11-15,$ and Year $16-25$ using the EAR formula.

$6.$ Average Effective annual interest rate of this $25$ years mortgage.