Face to Face Meeting
If you can arrange one, an in-person meeting with your representative and/or a relevant member of their staff is likely to be the most effective method of contact. A meeting usually needs to be arranged at least a week (and often more) in advance. Unless you're in Canberra, your best option is to try to arrange a meeting outside Parliamentary sitting periods. See when Parliament is sitting at the Parliament House Events Calendar.
Phone calls to your representative's office (local electorate office or Parliament House office) can also be effective, particularly in terms of expressing an opinion on a topical issue.
Letters (and even faxes) remain relatively effective methods for contacting Parliamentarians and will likely generate a written response, though this can take weeks if not months in some cases.
When writing to Members of Parliament, please refer to 'How to address senators and members'.
Contacting Parliamentarians by email is a fast but not necessarily always effective method of contact. Many Parliamentary offices now get such high volumes of email that it is impossible for them to answer them all.
Email and Letter
An effective approach that will greatly increase your chances of receiving a response is to send both an email and a hard copy letter by post. If you are contacting a Minister, this is particularly recommended, and you should also send a copy to the corresponding Shadow Minister, ensuring that you note this in the address portion of your letter. Letting the Minister's office know that their opponent has also received a copy will ensure it gets appropriately prioritised. This also gives you the opportunity to approach the Shadow's office if you get no response from the Minister's office.