Brit chick, the forms are all online and they are straight forward. It may take you a bit of time to gather up supporting information for your visa, but if there are gaps or information missing your CO should give you the opportunity to address these and send further information within a certain time frame. Our CO even gave us suggestions as to was additional evidence was suitable.
Everyones application is different.
You really don't need a migration agent unless you have a very complicated case!
Things you will need to read are
Which visa to apply for general info : http://www.immi.gov.au/migrants/family/family-visas-partner.htm
Brit chick if yours is the 309 you will prob be looking at :http://www.immi.gov.au/migrants/partners/partner/309-100/ <-- everything you need is in this one, including forms etc
Its really important you read this booklet and base your application on it http://www.immi.gov.au/allforms/booklets/books1.htm
Form filling
One for you (the applicant ) http://www.immi.gov.au/allforms/pdf/47sp.pdf
One for your other half (the sponsor) http://www.immi.gov.au/allforms/pdf/40sp.pdf
One if you have kiddies http://www.immi.gov.au/allforms/pdf/47a.pdf
Other info
* statements (one from you, one from partner) outlining how you met etc, when you decided to be "defacto", how the house runs etc, everything you need to include is in the booklet above http://www.immi.gov.au/allforms/booklets/books1.htm
*Statutory declarations should be filled out by family and friends http://www.immi.gov.au/allforms/pdf/888.pdf
Get them to outline your living arrangements, how you met etc and how wonderful you are as a couple. These need to be signed by a solicitor in the uk/ireland, although if you are in Oz any of the professionals can sign them off.
* Evidence of being a couple for a year- can be tricky see here http://www.immi.gov.au/media/fact-sheets/35relationship.htm
Include things such as
joint leases for houses, joint bills, council tax, wills or mortgage stuff.
Bank statements for both of you, at the same address for the past year (best send a month or 2 over if you can)
Joint invitations to weddings etc
a few (but not loads) of picture of you and the OT at events
- this isn't an extensive list - just a few ideas. Things will start to come to you when you start to put it all together!
* Your other half will have to provide evidence on how he can support you.
If you look on the really useful check list DIAC provide it should make this a bit easier to compile http://www.immi.gov.au/migrants/_pdf/309-100-checklist.pdf
Police check and medicals
* police checks are done through here http://www.acro.police.uk/police_certificates.aspx
I would wait until you are asked for them- they are quick enough at issuing them and it wont hold your visa up
* medicals cant be carried out until you have a number from your CO to book - one less thing to worry about for now
Phew, hope I havent put you off. Honestly it is straight forward if you read the booklet and send everything the checklist asks for .
Plus you have PIO forum - we've all been through it on here Good luck on your application!