Hairline cracking is normal (cracking less than 0.1mm in width) and can be associated with the plaster work drying out at the construction stage. The mortar around brickwork also can have small cracks which is quite normal.
Cracking that is more noticeable (but less than 5mm in width ) is also not a real issue and normally occurs over or around windows and door openings. Cracks this size are easily filled and will probably re open seasonally, due to shrinkage of the ground when drying out and close up again in the wet season from the ground swelling due to excess moisture. This is of course dependent of soil type.
Larger wall cracks (cracking 5mm to 15mm) do require some maintenance work depending on the locality of the cracking. Any localised cracking with multiple cracks in one area, such as above a doorway should have the loose brickwork removed and re instated to rectify the issue. With wall cracking up to 15mm, windows and doors may require maintenance and adjustment to open and close. Weatherproofing around windows may be an issue
Extensive cracking (15mm to 25mm) may involve replacement of wall sections and slab underpinning. Walls may be bulging with serious house structural issues. A report should be obtained at this point by a structural engineer if you are still wanting to purchase the property. Most underpinning companies will have a report written up by their engineer that you can purchase prior to deciding on what course of action you take, this report will however cost around $1,500.