Gold sovereigns and half sovereigns are gorgeous coins. All of them have portraits of reigning British monarchs on the front, and usually a crest or St. George slaying a dragon on the back. A few sovereigns, like this one, have a wreath around the name of its country of origin: Australia.
All these coins are valuable, especially the half sovereigns. Value is a combination of basic gold value and collector premium, as explained below. Here is pertinent info:
HALF SOVEREIGNS: 19 mm diameter, 0.1177 troy ounces gold
SOVEREIGNS: 22 mm diameter, 0.2353 troy ounces gold
To find an approximate value of your specific coin, first multiply its gold content by the current price of gold. This gives the basic value, BV. For instance, the BV of a half sovereign at today's gold price ($1474 US dollars per troy ounce, look it up for current value: kitco.com) is 0.1177 x 1474 = $173 US dollars. Second, estimate the collector premium using the listings below and add it to the BV.
HALF SOVEREIGNS, MOST DATES (*see below)
worn: BV + $100 US dollars
average circuated: BV + $250
well preserved: BV + $4000
fully uncirculated: BV + $15000
SOVEREIGNS, MOST DATES (*see below)
worn: BV + $10 US dollars
average circuated: BV + $50
well preserved: BV + $800
fully uncirculated: BV + $5000
*The listings above cover all dates except those below. The value after the date below is for average circulated coins.
HALF SOVEREIGNS, GOOD DATES
1855: BV + $15000
1856: BV + $2000
1860: BV + $2000
SOVEREIGNS, GOOD DATES
1855: BV + $5000
1856: BV + $5000
1860: BV + $300
You can see that some of these coins are worth a ton of money. If you have such a coin, congratulations! You should consult with a knowledgeable coin dealer or collector and have your coin authenticated, graded, and encapsulated by a third party numismatic (coin collecting) service. Use PCGS, NGC, ICG, or ANACS for this purpose. Do not use other services.
A great place for more information on sovereign coinage is Chard in Lancashire, UK.