Its vey easy to travel on a rented motorbike in Thailand. Northern Thailand is the most interesting place with the most interesting roads. There is mountains, fantastic scenery, cheap accomodation almost everywhere, cheap and tasty food everywhere, plenty of gas stations. Local people are in general very friendly and helpful. The only negative thing is that thais in general dont speak much english. But its strange how much ca be explained by hands...
The traffic can be a little overwhelming in and around big cities of course but as soon as youre out on the countryside you will probably love it.

Head to Chiang Mai first as there are plentiful of shops that rent out motorbikes. There are also one or two in Chiang Rai. I am now talking about bigger bikes from 250 ccm and upwards. Smaller bikes like 100-125 ccm for rent can be found almost in every town in the north.

Dont plan to ride 5-600 km a day. Ride 150-200 km a day and stop and look at the local sights and talk to the locals. Do a little bit of reaserch before you set off. Eg - read a guidebook. Yeah - and read also Loneriders tripreport. He did motorbike travelling the way it should be (in my opinion).

There are only third party insurance for motorbikes in Thailand but talk to the shop you rent about that. I will strongly advice you to have a decent travel insurance and make sure it covers motorbike riding and things that can happen related to that.

April will be steaming hot in Thailand but as Ausssies I guess youre used to it (unlike me...)
Remember also that the thais celebrate their new year, called "Songkran" april 13-15 and some places even before and after those dates. This means that there will be mayhem in the towns and along the road. Accidents numbers will rice because of a lot more traffic than normal and a lot more drunken driving than normal. So consider staying off the roads those dates and just join in and celebrate with the thais. Most foreigners love it....

Good luck with your travel!

If you need advice for where to rent bikes and where to travel - just pm me. I am fairly well known in northern Thailand. And I will also be in Chiang Mai in april.

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In the end everything will be fine. If its not fine its not the end....