Gold Coin Collecting

I bought a huge lot of coins yesterday. There are two different sizes of one NP coins and two sizes of 2 NP coins. Can somebody explain why?
There is another large coin, about the size of the two shilling coin which has the number 20 on the reverse, along with some sort of swirly design with an arm coming up from the bottom. Would somebody explain what that is?
OOOPS My mistake. I was thinking of the 5 and the 10 pence and also did not notice that there is also a coin which reads "one penny" but also has a large 1/2 on it. I was not expecting that.
I was very tired last night when sorting out these coins and missed a lot of things that I can see in the daylight.
'Nuther ooops--the coins with the 20 on them are from Australia